The mark to market value is the difference between the locked in value of a forward contract and the current market value of the contract, discounted for the time remain in the contract.
Vt = value of the forward contract at time t (to the party buying the base currency), (t < T) denominated in price currency
FPt = forward price (to sell base currency) at time t in the market for a new contract maturing at time T
FP = forward price specified in the contract at inception (to buy the base currency)
days = number of days remaining to maturity of the forward contract (T − t)
R = interest rate of price (denominator) currency