For paying back tuition debt, a simple estimate is 10% of your gross salary. Although, it's really a matter of what sort of life style you want to live.
If you make $80k/yr, take out 10% for debt and 40% for taxes. That leaves you with $40k/yr for all other expenses like housing, food, and so forth. Considering $40k/yr is the median pretax income in many places, this isn't exactly living like a pauper.
So, if you expect to be able to pay $8k/yr in debt repayment, you can then estimate how much debt you'd be willing to accept. If you take out $60k in loans @7% interest and have a 10 yr repayment schedule, you will end up needing to pay back $83.6k; roughly $8k/yr. (estimates from http://www.finaid.org/calculators/scripts/loanpayments.cgi
Let's get back to the $40k in desposible income. Would you be willing to use some of that for loan repayment? If so, you could assume more debt or pay off the $60k example earlier to reduce the amount of interest paid. If you looked to pay it off in say 8 yrs ($10k/yr), you would save yourself about $5k in interest charges.
Anyway, I hope that helps a bit.