Manually calibration wasn't inaccurate, it's just that many players couldn't be bothered to learn how to do it. The way I remember it, what really drove people out of bombers to jabos was the wind effects starting at 15,000ft. Basically bomb dispersion started to grow at 15,000ft, and got worse with altitude. So a lot of people simply wouldn't climb above 15k in a bomber, because above 15k level bombing was much less effective against point targets.
So the choice presented to the player became A: Take level bombers in at 14k and probably get shot down. B: Take level bombers in at high altitude and probably miss the target. C: Take a fighter-bomber instead. D: Fly a heavy bomber as if it were a fighter-bomber.