Pooks, for what it's worth:
here is the rivendell sizing method:
http://www.rivbike.com/how_to_pick_y...g_a_frame_size
I am 5'5" with a PBH of 80cm, and I take a 53-54cm Rivendell.
NOT the same system for other bikes!

All brands are slightly different, and no one can say Oh you are 5'8" so you need a size ___ frame. It is not the same from brand to brand. It's only a START to begin with standover clearance- you will need to address reach/toptube length as well, for one thing.
Glad you liked the Jamis steel bike!
You are doing great and learning more every day- that's wonderful! The more you know when you do actually buy your bike, the better choice you will make.