It looks like it's time consuming to do, but it takes less than 15 mins to do (small time investment before you part with your hard earned!).
Top tube length is equally as important as the seat tube length, you are only just shorter than me, I'd never consider buying anything bigger than a 54cm.
I'd say a 52cm would be more suited to you, the best way to find the right size bike is through a sizing site like:
http://www.competitivecyclist.com/za/CCY?PAGE=FIT_CALCULATOR_INTRO.
It looks like it's time consuming to do, but it takes less than 15 mins to do (small time investment before you part with your hard earned!).
Top tube length is equally as important as the seat tube length, you are only just shorter than me, I'd never consider buying anything bigger than a 54cm.