What was the approximate optimum bombing height for a 4 engined heavy bomber bombing say a factory to get the bombs on target? And for a 2 engined bomber?

What was the main determinant of actual bombing altitude in WW2? eg the 26,000' of B17s, the 18,000' of Lancasters at night, the 7000' of Marauders etc: was it the weather, winds, engine performance, fuel range, flak, fighters, bomb site performance?

How was it calculated?