How about these, possibly over complicated, ideas?
- A sliding FoW modifier: Start with a modifier, like -1 for every 50 units over 50-75. Every x (8 or 10?) activation rounds add 1 to the dice roll. Max modifier of +1, natural 18 always ends the turn. Might be the simplest. Problem is we tend to lose track of activation rounds.
- A modifier based on the number of enemy units. -1 for every 25 units your opponent has over 25-30. This may give an advantage to the smaller/losing side. Smooths the overall chances, would end more often after the larger side's activation. Also makes recombining a bit of a strategic question.
- Have FoW cancellations: Each side may cancel a FoW result a specified number of times. Perhaps 1 for every 50 units over 50 in the game. Either side may cancel a FoW roll, but inactive player gets first dibs. Cancellations available reset, perhaps every 3-6 tuns. Could make for some bluffing. Would you ant to cancel? Would you try to maneuver you opponent into canceling?