The range on the WGR rockets is exactly 1.4k. I tested this offline some time ago with 110s. The mortars drop about 500 feet by the time they reach their targets, as well. So usually you need to have them under your nose slightly (out of view) if you want dead-on hits.
In a 190A you can hit PgUp as far as you can, and where your cowling stops is where the bombers should be (between the bumps for your MGs, where sky meets cowling, put the bomber there and fire).
Of course that's for level flight and from the rear. If you're diving down at a higher speed fire further away and lead a lot more (like a CV 5")
Oddly enough, supposedly they were angled to use the standard gunsight, so that you could just aim it normally, but in AH this is not so.
BUT, if you sit on a runway not moving and you fire them, the puff of smoke will detonate EXACTLY in your crosshairs. Because it's firing up then landing down. If fired level it only goes down. I'm not entirely convinced the angle in AH is accurate.