If you look at the reply just before the one you want to refer to, say, in a link, along the bottom of the message there's a bunch of links: Reply Top Bottom Next Previous Edit
Rightclick on the one that says previous, and then choose to copy the link to the clipboard. That link will include the reply number in it, saving the need to count which reply it is in the thread, which can be a pain in the longer threads.
JW