For a level top tube bike like your felt, you generally wouldn't measure to the top of the seat tube (where the seat post goes in), but to the point where the top of the top tube intersects the seat tube. This is called a Center to Top measure. Some bikes are measured to the center of the Top Tube - Center to Center. That's more of a Euro thing.

The best method is to find the geometry chart for your model year and measure the Top Tube, which is always Center to Center. Your bike is older than 2005, the oldest catalog I can find, but you might use the F-series geo chart on the last page - it is likely to be similar (C. TTL - Horizontal):
http://2010.feltracing.com/Resources...05Road_Cat.pdf


If I had to guess based on looks (and you measured to the black seat post clamp), your F45 is a 54cm.

This is what a 56 looks like. Check out the gap between top and down tubes at the headtube compared yours: