Valuing FX Forwards

Hi all, Came accross some confusion when revising Schweser Mock 1 PM, Question 119. The FX forward valuation formula used is different then what is referenced in the derivatives chapter (where both countries interest rates are taken into account when discounting.) Sure enough, If you compare page 260 of Schweser Book 1 (econ section) the formula for the value of a fx forward is different than the formula quotng in Schweser Book 5 page 29 (derivs). Anyone encounter this or know if there a hard set rule for when to use which?

It seems as though when give a new set of fx rates for different maturities (page 260 schweser blue box example) you used the simpler of the 2. Appreciate any help

I wrote an article on this:

Once you account for the difference between nominal rates (Economics) and effective rates (Derivatives), they’re mathematically equivalent.