Sorry to be the bearer of bad news but you're not right. The interest is earned each year and must be paid on top of the principal payments that will pay down the loan. The formula is a little bit more complicated than that. It would be:

Payment = P*(I/12) / (1-(1+I/12)^(-n))

Where P is the principal loan amount, I is the APR interest rate, and n is the number of periods.

In your case the monthly payment would be $1672.88

The best way to figure out payments is to use a financial calculator or you can use Excel. The formula for use in Excel would be

=PMT(0.08/12,240,200000,0)

or =PMT(I,n,P,FV)

where P=principal, I=Interest per period, n=number of periods, FV=final value.

Good luck.