Calculations in Acrobat only kick in when a value is committed, which happens when you leave the field. This can't be changed.
In theory it might be possible to use the keystroke event of a field to change the value of another field, but that is going to be quite complicated and not really worth the trouble, in my opinion.
See this tutorial about setting up advanced calculation in Acrobat: http://acrobatusers.com/tutorials/how-to-do-not-so-simple-form-calculations (although it seems to be offline at the moment)
If you have a specific question after reading this tutorial post a follow-up here.
You would normally set all calculated fields to read-only so they are not able to receive the focus and so the user is unable to attempt to change their value. The problem you've having will most likely be fixed if you set the field calculation order (which is not the same as the tab order) to whatever makes sense for the form.
If you have the outstanding balance before a given payment is made you should be able to compute the amount of interest for that payment. With that amount and the periodic payment amount you can get the amount of principal being paid by subtracting the interest paid for the period from the periodic payment.
Thanks for the replies and help. Setting the calculated fields to read-only makes a ton of sense, so i will make that change.
The Excel file i'm trying to replicate is here and the button I need calculated is G28.
Loan principal may be found using this formula to mimic Excel PV function
PV = -FV * (1+RATE)^-NPER - PMT * (1+RATE * type) * ( 1- (1+RATE)^-NPER ) / RATE
You may also need to round the computed values or the breakage or fractional amounts less than 1 cent. If no adjustment is done an error will accumate in the repeated calculations until it becomes obvious.