Using cash fares to value Intl F awards only applies if you would purchase those otherwise, or if you really think that the F experience is $18000 better than the Y. (Hint: if you're in F on an award and the GA ofered you $15000 cash to downgrade to Y both ways, would you take it?)
But it also depends on how often you travel on your own dime and how many miles you have stashed away. If using the miles now means that you won't have enough left for that trip next month that might price at $500, I'd save 'em. If you have a ton stored up, burn 'em. My own personal threshhold is between 1.25 and 1.5 cents per mile; at $376, I'd probably use the 25K instead.