RPG math

Hi all,

I recently asked how to access an EXCEL formula from an RPG program
but here I am now having to reproduce the EXCEL PMT function in RPG.
Maybe someone has already done this?
Anyway, I've been given a formula to code and an EXCEL spreadsheet to
control my results.

Which I've coded like this :

MonthlyRate = interestRate /12;
     MonthlyPayment = ( LoanAmount / MonthlyRate ) *  ( 1 - ( 1 +
MonthlyRate ) ** ( -1  * numberOfPayments ));

LoanRemaining =
12 * MonthlyPayment *
1 - ( 1 + MonthlyRate ) ** ( -1 * numberOfPayments -
numberOfPaymentsMade ) /interestRate ;


My fields are defined so (imposed) :

LoanAmount, MonthlyPayment, LoanRemaining 13S2
numberOfPayments, numberOfPaymentsMade 3 I 0


I've very little experience with RPG math and I'm worried about errors
from intermediate calculations
How should I define the rates MonthlyRate and interestRate so I don't
get any loss of information after the divisions?

Thanks.