If the target is going slow, you'll want to shoot about 1.4k or 1.3k. I did some tests way back with 2 110Gs (one flying straight, other shooting, trading off when it was out of rkts). When they're going about 150mph it's 1.4k
However, at alt, when shooting at 300mph bombers, I find you have to get in to at LEAST 1k. You also have to shoot high, as they drop considerably.
Coincidentally, I got a kill on a lanc last night with 'em. Probably the first in many years. Granted I stopped USING them for years, but still!