A "good source" for a discussion of the optimal tactics for medium bombers to avoid flak is Joseph Heller's Catch 22, for example the discussion on evasive action between Yossarian and McWatt. Although the footnotes are not up to this forum's high standard, it is worth noting that Heller had flown 60 missions as a B-25 bombardier.
ps. For those who don't know the book
"Bomb bay clear," Sergeant Knight in the back would announce.
"Did we hit the bridge?" McWatt would ask.
"I couldn't see, sir, I kept getting bounce back here pretty hard and I couldn't see. Everything's covered with smoke now and I can't see."
"Hey, Aarfy, did the bombs hit the target?"
"What target?" Captain Aardvaark, Yossarian's plump, pipe-smoking navigator would say from the confusion of maps he had created at Yossarian's side at the nose of the ship. "I don't think we're at the target yet, are we?"
"Yossarian, did the bombs hit the target?"
"What bombs?" answered Yossarian, whose only concern had been the flak.
"Oh well," McWatt would sing, "what the hell."