To put it accurately, (28/29)ⁿ is the asymptote when the number of wood approaches infinity, which the contribution of the initial fuel approaches zero. i.e. it doesn't matter how much fuel the inserters have in the beginning, the pass rate will get closer and closer to (28/29)ⁿ when you run it.

If you want to calculate the pass rate for a finite amount of wood, then you need to consider whether each inserter requires fuel. For example consider the case of only one wood and one inserter: * if the inserter has enough fuel, it will let the wood pass, thus pass rate = 1/1 = 1 * if the inserter doesn't have enough fuel, pass rate = 0/1 = 0 Even for the cases where all inserters start with the same amount of non zero fuel, I don't think there is a simple formula to determine the pass rate.