One shouldn't make such a calculation the back of an envelope. One should contact a legitimate mortgage lender, sit down with them and have an intelligent conversation. Backs of envelopes are for doodling, jotting down phone numbers, perhaps making a grocery list. Buying a home is a major financial decision.
Secondly one should not confuse what one can afford with what one may really need. I can afford more house than I currently live in as can Warren Buffet for example, yet neither of us chooses to buy more house than we need. Living well within one's means pays long term dividends.