How much of a house can you afford?

Many experts say that your mortgage payment (principal+interest+tax+insurance) should not be more than 25% of your gross income. If you listen to the likes of Dave Ramsey, that mortgage should be no more than a 15 year fixed rate. But, I was looking at what different salaries could afford (maximum) as a 30 year fixed rate under this plan:

$40,000 salary - $122,000 mortgage ($833/mo)
$60,000 salary - $183,000 mortgage ($1250/mo)
$80,000 salary - $244,000 mortgage ($1667/mo)
$100,000 salary -$305,000 mortgage ($2083/mo)

If you change it to a 15 year fixed as Dave Ramsey suggests:
$40,000 salary - $90,000 mortgage
$60,000 salary - $135,000 mortgage
$80,000 salary - $180,000 mortgage
$100,000 salary - $225,000 mortgage

The assumptions I made were that the interest rate was 6%, taxes at 1%, and I used this calculator which adds in insurance. This assumes also a 20% down mortgage with no PMI. That mortgage calculator incidentally doesn't allow you to go over 28% of your gross, which isn't that much more than the 25% rule. It also allows you to indicate other debts.