one correction

rental income (50% or 70%) is added to income OR (not "and") deducted from debt.

 

A lot of banks have mortgage payment calculator, by which you can find out the monthly mortgage payment for your next mortgage.

And also a lot banks offer something like "how much can I afford" to calculate the max amount you can borrow. Of course, these tools often don't include your rental income in a way when the bank calculates your GDSR/TDSR