covered interest arbitrage - formula, please? currency, 120, interest, practice, country

CFA4Techie

I seem to routinely get the wrong answer on covered interest arbitrage, and I'd like to memorize a formula, rather than a whole series of steps (borrow in this currency, sell short this currency in the forward market, etc). For instance, given:

Rb = interest rate in country B(ase)
Rc = interest rate in country C(ounter)
S = spot in B:C
F = forward in B:C

I've seen one answer in a Schweser practice exam (#120 on exam 2) essentially compute:

(1+Rc) - (1+Rb)(F/S) and multiply this factor times the notional amount in C's currency.

But in the Schweser online class, essentially the formula was:

(1+Rb)F - (1+Rc)S and multiply this factor times the notional amount in B's currency.

I knew that in the process of typing this question it would finally start to come together. C's currency is related to B's currency by a factor of 1/S, and the formulas above are equivalent multiplying through by -(1/S).

Well, I wonder if anyone else has a helpful mechanism to remembering how to solve this sort of question. May only be one question exam day, but I might be taking Level 3 this June but for one or two questions