oes this frame fit?
 

(should have read, Does this frame fit?)Right now I ride a 57cm Bianchi compact frame. The virtual top tube length is 56cm and the seat tube length is 54cm. I also have an old Centurion traditional frame that feels comfortable to me as well. The top tube length is 57cm and the seat tube length is 53cm. In both cases, the total of the two equals 110..could that be the reason they both feel like they "fit"? I am just about 6 feet tall with a 32" inseam.

yes.

The total of the two has nothing to do with it, but a picture - with you on it, in riding position with the cranks parallel to the ground - would be required to make a judgement with any accuracy at all.

When I fit a new bike to myself, I try to get the following measurements as close as possible to those of old bike:
1) Length from the bottom bracket to the top of the seat.
2) The fore/aft location of the saddle, as compared to KOPS
3) Length from the saddle's "sweet spot" to the handlebar.
4) Saddle to bar drop (optional)

Of course, they won't ever be exactly the same, and the most important part is that the bike is comfortable.

The ST length has no effect on the fit at all. All it does is change the amount of exposed seat post.

What is really important is the frame REACH. Reach takes into account the TT length and the effect of the seat tube angle. Two frames can have different TT length but fit the same, if the STA difference happens to cancel out the TT length difference.

Unfortunately, there is more than one definition for reach. You can get about the same information by comparing the TT lengths, with proper compensation for any difference in the STA. The compensation for STA is 8-10mm per degree, depending on the exact frame size. Steeper STAs make the reach longer.

If your frame with the 56cm TT has a 1 degree steeper STA than the other frame it would have a nearly identical reach, compared to your other frame.