The Old Mortgage Lending Standard

I was trying to explain to someone about the housing bubble but I needed some specifics. We know that there is a bubble but I was looking for how to determine one mathematically.

Basically I started with the old lending standard. A buyer would needed 20% of principal and then would finance the rest over ~30 years at a certain interest rate. If I remember correctly, the total amount (principal and interest) would up to be about 3X the original principal of the home when all the money is finally paid up.

Then I needed to remember how the median income level of an area relates to the median home price. I think it used to be that a buyer would pay about a quarter to a third of his monthly salary on his mortgage and that was the general standard.

Can anyone help with this? I tried looking it up but the web is impacted with subprime garbage info.