You can go the measurement route, very sensible.
I did mine a couple of years ago, on the wrenchscience web-site.
I think there is no substitute for setting your best feature up on the saddle.
You'll learn more by checking the fit on the bike you're considering purchasing.
There are alot of variables, and you really should take a long ride too.
How the bike feels in the shop may not be how it feels after a long day in the saddle.