I am curious, if you have time could you measure from the bottom bracket (that's where the pedals attach) to the top of the seat tube (where the seat post goes in-- don't measure the seat post itself)? That's where they got 15" on the Trek you tried. Your height & measurements are pretty close to mine and I ride a 17". My old bike was 15" and it was too small. I made it work with an extra long seatpost and some other adjustments. I can ride my dad's 19"-- with a lot of adjustments.

The bike size, and the adjustments the bike shop make, will get you pretty close, but I found it was just a lot more convenient to make these tiny adjustments on the spot myself.

It is important to tighten things down really well. My seat slipped more than once-- but since I knew how to adjust it I could get it back into place easily. It was pretty funny when it was saddle angle I was tinkering with, and I'd get it where I wanted and then halfway home I'd get goosed when it slipped!