Steve,

With your formula, I think you would have to use something like:

{ =(P*r)*((1+r)^n)/((1+r)^n-1) }

Rember r is going to have to be converted from APR. I.e., 6% will need
to be changed to .06/12 or .005.
You could also use the shorter formula:

{ =P*r/(1-(1+r)^-360) }