Amortization and Annuity Payments
The pro version of the plugin offers the option to calculate repayments using the standard annuity formula:
P = rV / (1 – (1 +r)-n)
P = repayment amount
r = interest rate per period
C = Current loan amount
n = number or repayments
To calculate the rate per period you need to convert the APR.
If you want to use a different formula get in contact and I will see what I can do.
For more information on the annuity formula take a look at the Wikipedia page.