GDSR is monthly mortgage payment (including future one)/total monthly income. It should be less than 32%
TDSR is your total monthly debt payment (including car loan etc, and future mortgage payment)/total monthly income. It should be less than 42% or 40% (vary by banks).
Your rental income count 70% or 50% depending on banks added to monthly income, and deducted from monthly debt (also depending banks).
For your current house, since it ha no rental income yet, you can ask the bank to do an estimate (or appraisal) for how much it can be rented.
The future mortgage monthly payment is calculated by 5-year fixed post rate, not the actual term and rate you will get. Usually, banks or mortgage brokers base on how much you ask for to do the calculation, then decide you are qualified or not, not the other way around to calculate how much you can borrow. better talk to a broker or your bank.
