Stat bonuses are almost always based on your base stats (natural stats + mods + DC).
So it would be 50,000 x (1 + 0.4 + 0.5) or 50,000 x 1.9 (technically, I believe each boost is calculated separately and rounded down, but that's getting in the weeds a bit much for HP calcs, though it can matter a lot for speed)
I don't have an official link. You just have to test these things out for yourself, unfortunately. Some are easy to test. For example, take some characters into GC and bring Hoda along. Use his basic. Check the numbers. For speed calculations, do some tests while moving mods around if you aren't sure exactly what your speed will be.
