This afternoon, I did an actual measurement of the standover height on a 50cm Aurora we have in the shop, measuring as close to the center point between the seat tube and head tube as I could in just eyeballing it. The height to the top of the top tube is just a tiny bit over 30 1/2"", perhaps 30 5/8." This thread got me curious as to the actual height of the top tube at the middle point, compared to the stated standover height on the Jamis geometry chart. This was with 700 x 32c tires on the bike.

imi makes a good point about barefooted inseam measurement (if that's the way you measured it) also. I really wouldn't worry about it too much anyway, based on the info you've given I think the 50cm is the size for you if you want a Jamis Aurora.