computational effort needed to mine a block given the number of leading zeros in the resulting hash? What is the computational effort to mine a block successfully given the number of leading zeros in a resulting hash?
The difficulty does not specify a certain amount of computational effort that needs to be expended. In other words, the difficulty is not set to '1000 effort', and miners slowly progress from 0 to 1000, with the first one to expend 1000 effort winning the next block. 

Instead, you need to think of it like a lottery. You can expend effort to make a guess, and if your guess is correct, you win the next block! The higher the difficulty, the lower the odds of any one guess generating a valid block.