[PMoz99]

Stig, my initial info came from Wiki - yes I know it's not that reliable but it was the quickest source I could find. I've seen other online sources quote similar figures - B-2 range 500km with the R-2 adding 360km with the additional tanks. It's possible the various sites may have info from the same source, as sometimes happens, and if it is in error then it's just being compounded.
The Squadron/Signal Ju-87 in action publication quotes the same figures as you have for the R-1 and R-2.
Achievable range would generally depend on the load, cruising speed, and height, and in individual cases prevailing winds.
I haven't heard of the rule-of-thumb adjustment you mention, but I don't profess to be an expert on Ju-87s. I guess in the end it would all depend on the bomb load carried.
Cheers
Peter