I seem to recall that my 2 Quantums were measured - and marketed, as the TT length being about 1 cm larger then the ST length. Thus my 54 has a 55 ST. Note that these were level top tubes at the time (late 80's and then a '94).

But you should always get the effective TT length if a sloping tube, or the TT length if not, as that's pretty much the determining factor.

So if you are currently riding an effective 56.5, then a 55 or 56 would be withing the range of reason as both are readily accommodated using an appropriate stem to get you to where you are now.