Optimum bombing height

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?

[Tony Williams]

The optimum altitude wold be as low as possible (moderated only by concern about self-harm from the exploding bombs). The higher up you go, the more the chance that cross-winds would blow the bombs off-target.

The main reason that bombers usually flew as high as they could was to avoid Flak, and also I expect to make the defending fighters work harder to climb up to their altitude (the higher a bomber flies, the greater the performance advantage needed by the fighters to make an interception - and the sooner they would run out of fuel).

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