Looking at that chart, and looking up some of those bike's geometry's has popped a question in my head. Some people were quoting their inseams, then i go to check the standover height and was often taller than their inseam by .5-2 inches! I don't see how that would be very feasible when doing any recreational trail riding.
Anyhow, comparing the specs to many of those bikes listed for those around my height and inseam and the ideal TT length, I think the 56cm TT length would be okay for the RCX and I am seriously considering buying it in the next few days.