Most of the waiting here is not gun powder. The low grade fuel takes longer than any other step in the chain, and each bomb drinks a real pile of it, fuel your lanterns would otherwise burn. The upside is that fuel needs no bench, so you can craft it from your inventory anywhere, even on the way out to the siege.
Rope cannot be crafted at all, only looted or recycled. The calculator counts it as is, two go into every bomb and the catapult itself took one, so grab every rope from barrels and crates in the days before. Fuel batches suit pairs of bombs: one bomb leaves a sliver over, two use them exactly.
On sulfur this is the light round, a third of a propane bomb, and that is the right price against wood. Stone barely notices the fire. And if the problem is your sulfur box rather than the wall, a bee bomb costs none at all.
