I’ve never been a fan of strategic bombing raids, because on average it does not pay off and your bomber can do more damage elsewhere while attacking units in the long run.
The chances to inflict 12+ damage in 3 subsequent raids is 37,6%
The changes to lose your bomber to AA in this 3 raids are higher: 42%
Of course you could do more then 12 damage but in average its only 10-11 damage you would inflict.
This changes once you have access to heavy bombers, but this is rarely the case.
How about this more simplistic rule for strategic bombing raids:
On a roll of ‘1’ by the AA the bomber is repelled and returns to its origin. No damage taken by either side.
Optionally, if using the Bomber for no effect is not threadening enough:
Aditionally the bomber is marked as damaged and need to skip its next turn to repair it for free.