don't pay attention to your inseam imo. i think most people measure the size C-T as you stated above. if you are going to buy a frame online, you need to ask the seller how he/she measured it or ask for a C-T measurement.

its really about how you want to feel on the bike. a larger frame will allow you to have your bars close to seat height without a goofy stem. a shorter frame will be lighter and more compact (if the top tube is shorter)...

i would measure the crap out of your bianchi and decide what you like and don't like and go from there.