How big a mortgage?

What is the best way to calculate the most you should borrow to try to insulate yourself from financial difficulties going forward? I know the banks have online calculators to indicate how much they would lend you, but what would YOU say is the best way to figure out the most you should borrow;

Should your mortgage repayment be less than 40% income if you use a 25 year term and assume 6% or 7% rate. Therby leaving room for rate rises, pay cuts, rates, property tax etc. and allowing room to extend mortgage duration if required.

Appreciate advice as don't want to bite off more than I can chew!!
Cheers

Personally just add up all your outgoings each month. Then take this figure away from what you bring in.

This new figure is what most people think they can spend on a mortgage, but I would personally go a lot lot less. The rates are only going one way, and its going rapidly.

If you end up with disposable income after your car loans, credit card bills, Sky TV, entertainment, child care etc etc etc that say is €800 per month, I would not take out a mortgage for anything over €500. After all, with a mortgage you also have to budget for life assurance, illness & unemployment, critical illness, house insurance, contents insurance, plus decorating, furniture and heating the thing. It all add up and I don't really thik you can put a % figure on what you should spend.

You will know yourself. Don't stretch yourself at this early stage or you may pay dearly for it later. Just ask one of the few hundred thousand who are in that boat.