I measured that way. My inseam is 90cmx.883=79,47cm. So I set the seat at this height, numbness is gone with the Romin seat but not with the san marco that came with the bike.
With the romin now, I only get soar. But I do get a little pain in my lower back, dont know if it comes from my old bike (wich was waaaay to big) or if its my body needing to adjust to the new one.

I tried to lower my seat about 1,5cm now but I havent tried it since I had a workout earlier today. If you take a look at the thread called "Bianchi Nirone 7 or Bianchi Impulso" I have a few pics there, with the seat before I lowered it. Its also with the seat that came with the bike.