Widewing, this works for me in P-38 (though it still doesnt work most of the time

)
Set salvo to 10, delay to .2 (.20). Put your plane directly in front of the buff's nose and with either 5k alt adv or disadv.
When buff is d3.0, split-s or climb, TRIM the plane so the vector is steady and fire the rockets 2 buff lengths in front of its path.
The delay in the fire of the rockets will give you a better margin at having 1 of them hit.
Another trick is to power dive on the buff from high 12 oc and fire the rockets at the buffs nose when below d1.0 while corckscrewing in the direction of the buff's path. That gives you a nice wide pattern of rockets flying in the buff's path.. more chances to hit.
Problem is the compression hehe. Wish them dive flaps worked *G*