Vdubchaos,
Your C point is going to be different on the area people live. I live in a state with property tax, but no state income tax. If I lived in a state with no property tax, that 30% of your take home pay per month for a mortgage payment would be spot on. With property taxes, that is a bit different. My mortgage payment per month is right at 30%, but my property taxes are then on top of that. So I had to do a summation for total out of pocket I wanted to spend. If I did taxes + mortgage at 30% of my income. and decided to try a buy a house in San Antonio city proper (which has highest income taxes around here), then I'd be living in some pretty crappy house in the not in best of neighborhoods either. Which would more than likely be an older home, with horrible efficiency and maintenance costs. Which would have pushed my over all living bill way up.
That's really not the best way to approach things. Best way is to take your net monthly income and decide how much you want of it to go to ALL bills. Do a budget of what you know such as food, phones, tv, student loans, car, insurance, or whatever. Add up everything you pay now minus your rent. Most people pay from 50% to 75% of their net monthly income to monthly bills total. So then look for a house within that range of your budget. I wouldn't recommend ever going over 75% of your income towards bills. You won't be able to save nor splurge very easily.
Here's how I looked at things when I went shopping for a house. I had a budget in mind that I didn't want to spend over $2800 in total a month for all my bills. I did up my budget and played around with what I know and what would be unknown.
Unknown because those factors would all be based on where and what I buy for a house. Older homes tend to be less energy efficient, have higher warranty costs, and have maintenance costs. I have to figure in tax rates for where I moved. I figured in HOA costs, school costs, and even gas for how far I would typically have to drive to these following common places: work, schools (when I have kids), and grocery store.
Take a friend of mine for example where I work. He makes about the same amount as me, but bought his house a long time ago. We compared our "budgets" together because he thought I over bought out of my budget based on my house.
His house is older, built in the 70s, smaller, has outdated amenities, and was cheap. The house is a 150K house. He lives in SA proper. His taxes are like 4%. I am going to assume the same 4.5% APR for both of us, although I was a bit lower on mine at 4.3%. His monthly mortgage + tax cost for his home was $1350 or so if I remember right. I have a 250K house, but 2% tax rate and no PMI. My Mortgage + tax cost is $1800.
Then we looked at energy bills. My average electric bill so far has been $150. His is $400. He has to drive 20 miles more into work than me. His mileage for gas is higher and pays an extra $200 on top of my costs despite being closer to a grocery store than me. I have a new built home and warranty for it was cheap. His not so much and he has maintenance costs I don't have to worry about for 10 years. He spends on average $3K per year after looking over his purchases on doing stuff for his home that I won't have to. Which is another $250 a month. Gas stations around me are cheaper at $2.96 on average per gallon compared with his $3.20. Cost of living and groceries are cheaper.
Sure the house I bought has a higher mortgage rate, but he pays way more overall for all his living expense bills compared to me. That's the crux of it. That 30% for monthly mortgage payment is not really that good of a hard-fast rule to follow. A person needs to sit down and do a proper budget. Look at ALL costs and decide how to buy. Sometimes you can buy more house if it lowers your bills in other areas based on location of the purchase.