Mark to market economics confusion

CFA Level II Economics
#1

Please help!

I see the numerator for mark to market is FPt - FP but in some practice questions, the variation of FO - FPt is used.

Also, for the denominator, how do we know which rate to use to discount? Sometimes it’s the base currency and sometimes the price currency. What is the rule?

Thank you

#2

Numerator is FPt-FP

FPt -forward price of at time t for new contract maturing at time T

FP- forward price specified in contract at inception

Denominator will always have interest rate of price currency