When using karl's Calculator for the principle do you put your current principle rather than the total amount your borrow, eg if original amount was 100K now down to 70K,just trying to figure out by keeping payments the same even though rates are going down and by transferring trs back of the mortgage aswell.On a tracker of 0.5. The rate of interest is on the original amount? so if I paid 10k of what does rate come off