To be fair, we can estimate a proper amount, but we're having to go by assumptions about the health and number of HEALTHY yeast, not simply the amount of stuff in the bottom. Plus there is who knows what kind of, and how much, trub mixed in.
Many pro brewers can get a pretty good idea on yeast count, but it's not so easy for homebrewers. I think it's much more difficult to overpitch, and get poor results, than it is to underpitch and get poor results.
If you are going from a beer as light as 1.050, and using it to ferment a beer as big as a RIS, I would RDWHAHB and just do it. You may be overpitching by a few billion cells, but unless you can inspect the quality of your yeast, and count it, you just won't know for sure. Not everyone wants to spend extra time doing the minute details.