There is no optimum firing range for any rocket. They are either HE (they rely on explosives to do damage), or hollow charge (H.E.A.T. IIRC. Meaning they don't rely on speed to puch through armor)
The PB1 rockets (190F8), and the rockets on the Il-2 and I-16 are the only AT rockets in the game. Obviously a flat impact is preferable.
I'm by no means an expert on rockets, but the 3.5", 4.5", 5" HARV's, and the Russian should be fired when your target is a bit infront of your nose (IDK about the range, but I guess D400, maybe). The German PB1 rockets need to be fired as your target is about to pass under your nose.
The WGr21 rockets have impact fuses along with a 1000yd fuze. That means they will detonate on impact, or when they have traveled 1000yds (1k, but the icon ranges aren't an accurate measure untill they show "1000" as opposed to "1k")