Depends on what type of guns too. Allied planes (.50 Cals, Hispanoes), are almost like lasers and have little drop in their trajectory.
MG151s, MK108s, and 13mms all have pretty bad ballistics and require more lead if you do not fire at the same range your convergence is at.
D300 is the optimum range almost every gun except maybe the P39s 37mm which unlike most cannons, have a very flat trajectory and can be set at D650.
My .50s are set at 350, my hispanos at 400, my MK108s at 200, and every other gun at 300.