You really shouldn't do that, you should just choose a speed preset and a CRF setting. First pick a speed preset you are comfortable with in terms of encoding time (the slower the speed, the better the quality/space ratio will be). After you've picked something that you're comfortable waiting for (don't bother with placebo preset...), pick a CRF that satisfies your quality requirements. Default for x264 is 23, but anything between 18-23 is reasonable for HD material, 18 being the highest quality but probably not much smaller than the original. 23 is good enough for grain-free or video that have a lot of still images, but for busier sources you may want to use 22 or 21.
And that's it, you can just encode over and over always using the same settings. The encoder will decide how much and where to put the bitrate to maintain that same perceived quality across the entire video.
This is the best way to encode with x264 (and x265). Two pass + target bitrate does not allocate bitrate better, it's actually worse.
I couldn't agree more. When I first decided to try this whole 'compress my optical disks' thing, all signs from different Google searches led me to believe that using certain bitrates was the way to go. And it
is the way to go, if you don't mind your Blu Ray movies looking more or less identical to uncompressed DVDs. If you are used to watching movies that actually look good for the entire movie, you'll want to go with the way that Jag and I do it, with the constant quality settings.
Here are the settings that I use for my Blu Ray discs, and on my tweaked 46-inch screen, I can see no difference between the uncompressed disc, and these settings:
1) Picture tab: Change only 'Cropping' to the Custom setting. Leave all four settings below Custom set to 0.
2) Filters tab: A) Detelecine: Default B) Decomb: Decomb @ Default setting C) Denoise:
hqdn3d @ Strong setting Leave completely off. Wasting a tiny bit of HDD space on a couple of DVDs of extremely old videos is 1,000s of times better than making all of your new stuff look comparatively bad. D) Deblock: Off E) Grayscale: unchecked
3) Video tab: A) Video codec: H.264 B) Framerate: Same as source, with Variable Framerate C) Quality: Constant quality @ 18. This will give you constant quality on every one of your Blu Rays, with shorter (85-95 minute comedies) being lesser amount of data. Since I only have a tweaked 46" TV, and you have a 65-incher, you may very well need to go down (up in quality) to the 17 setting. Expect 20-25% larger file sizes with the 17 setting, though.
BTW, the file sizes are pretty tiny. My largest file is 11.6 GB, averaging between 5 & 8 GB, with my smallest full Blu Ray movie @ 2.75 GB. The more action a movie has, the larger the compressed file will be, and the longer the movie, the larger the file will be. All four of the Shrek movies are 3.XX GB, for instance. All of my Blu Rays were encoded with the exact settings in the paragraph above and the paragraph below. All of my DVDs were encoded with those settings as well, with only the constant quality setting changed to 19, instead of 18. DVDs become as small as 500 MB, and only a few of them being much larger than one GB, with ~1.5 GB being a rarity.
Having a fairly fast "main" system, the 4.4 Ghz 4970k that is in my signature, I played around with the settings under the Optimize Video settings. You will get the best mixture of quality videos with the smallest amount of data from using the 'Slower' X.264 preset. Now, it takes longer than the settings to the left of it, but there is a discernible difference between them. I can personally even see a noticeable difference between the 'Slower' and 'Slow' settings, with 'Slow' ending up a larger file size. I (personally) could see no difference at all between the 'Slower' and 'Slowest' setting, even though the file size roughly doubled, and the encoding time went from ~4 hours to more than 20 hours.
The settings you will need for the bottom of the Video tab are dependent upon how robust your playback device happens to be. I wanted to make sure that I would have no problems playing back my files on any device that I own, including my 1.3 Ghz Atom tablet, so I encoded them using an H.264 profile of Main, and an H.264 level of 4.0. You'll have to either check around with Google, or just try out different compression settings for yourself, with your older WDTV device.
Lastly, the audio tab. For DVDs, you don't need to change anything. At least, I never needed to change anything. For Blu Rays, though, if you don't specifically tell Handbrake which language you want encoded, you will end up with gorgeous re-encodes, with zero audio tracks of any type! Assuming you and everyone in your household speaks English, you only need to 'Add' English to the right hand panel. Feel free to save a few GB (total, not per movie) by limiting which of the audio tracks to include, but I personally include them all. It doesn't add much data, and if/when my audio setup changes, my encodes will already be prepared.
Oh, one more thing. Make sure to save your settings! I have two different settings, one for DVD and one for Blu Rays. These are added settings, that will appear below the User Presets on the right hand side of Handbrake. Good luck.
edit: After you have everything all figured out, as far as quality settings and compression settings, you can then setup a huge 'queue' of movies to do one after the other, with no input needed from you. My 4790k takes between 2.5 and 5.5 hours per Blu Ray, with both being rare. Nearly all of my Blu Rays took between 3 hours and 4 hours, with some between 4 and 4½ hours. If your fastest computer is half as fast, it will take twice as long. If your fastest computer has twice as many cores/threads at roughly the same speed, it will take half as long.
edit #2: BTW, on my system, DVD movies take 25-45 minutes to re-encode, depending on their length and the amount of action, with a couple of the very longest taking right at an hour. Also, note that not using denoising will not only make your movies look better, it will also add at least
30-40% 20-25% more data to the encode, as I've discovered. Of course, that's the portion of the movie data that you did not
want to remove, so it's still a good thing.

Either way, the amount of data you'll need to store will be many times less than storing them uncompressed, as you have been.
See the post below by JAG87 for the reason not to use denoising on your digital movie data.