In the book "Under the guns of the Red Baron" - Norman Franks et al, MvR's combat reports are all listed ( retranslated ) and he often describes getting to within 50 and even 20 (!) yards before firing.

OTOH, a survivor of one of his kills also says how amazingly accurate he was in a diving attack - so even though he WAS a very good shot, he nevertheless did not fire from long distance unless he had to.