Mobil,
You cant bomb at 20' agl or 50' agl or 100' agl -
Two things happen when you are too low:
1 - your bombs dont arm - ie dont explode
2 - they arm and explode too close and kill the bomber
Personally, when I am in Lancs and bombing tanks (a personal favorite thing to do), I gotta get some dropped when I'm seeing the TIGR or PNZR at 600 or so.
And you really need to try flying some buff formations to get a feeling for how frustrating it can be. They dont synch up well, its a pain to manuver, good fighters can kill you fast.
I drive tanks a lot and buffs who can kill tanks are my greatest threats. When a spawn gets camped - after 200 or 300 deaths - ill up some lancs to bust it up

My best drop was 3 PNZRs and 1 TIGR in a single salvo of 1 from three planes - they were packed in a bit too tight for their own good - lol.
Anyhow enjoy,
Snak