An easy rule of thumb is:
The % change in payment (at least the interest part) is directly proportional to the % change in interest rate. So if rates are 4% and you raise the rate 1%, that’s a 25% increase in rates and a 25% increase in the interest payment.
If you go from 4% to 8%, that’s 100%, so your payment effectively doubles.
At least that’s what I figured out, if I’m wrong, feel free to flame me.
