Seat height….

Slow, is it possible you’re
Slow, is it possible you’re using the otherwise pretty accurate Lemond method but measuring the wrong dimension on your bike if your feet are flat on the floor? The inside leg x 0.883 Lemond formula is supposed to derive the bb centre to saddle top. Sounds to me like you’re measuring from the pedal spindle to the saddle top. If you want to use that dimension, multiply your inside leg by 1.09. That formula works pretty well, too, and it takes into account your crank length although that’s almost certainly 170mm on your size bike.

If you want to try something else and you have an iPhone try the SizeMyBike app http://www.sizemybike.com/ I’ve verified it with myself and about six other people’s fit and seems good; not least because it gives you all the required dimensions and not just saddle height.

By the way, avoid buying bikes by the nominal frame size. You say you’re thinking of buying a 54.5cm frame and that may well sound OK for your inside leg length but that takes no account of the top tube and it’s that that matters if you say you have a ‘long back’. It’s possible to make generalisations about frame sizes for average proportions but these days with sloping top tubes and the rule book for what the nominal frame size actually means being thrown out of the window by some manufacturers, it’s actually easier to just go by the effective top tube length.

Your 57cm top tube might just about be OK depending on your arm length and preferred position but I don’t imagine your stem length should be more than 10cm?