Combination of clipboard maps, a bit of trial and error, and a little experience.
You don't "need" a way to change the map works. You need to change the way you work.
First off pull up a clipboard map, have a straightedge handy, now look at your target, and think about which possible lines will give you the best results. A bit of trial and error offline for each base type will quickly put the finishing touches on your map work. Also figure out which line is best if you turn and hit it again. Note compass headings, landmarks.
Then its just a matter of implementing it.
Line up your attack, use your compass, its more accurate than the map. When the target is in long visual range jump either to external view or the front gun. Line up your bombers so they will overfly the target correctly. If you did the prep work right, this will give you a near perfect run. Once aligned, calibrate, zoom and drop.
If your smart, you'll take notes. Its not hard to be a very good bomber pilot, just takes a little attention to detail.