I am 6' tall. Which means by Saddle Ups post I would fit a 56 or 57cm.

But its unclear whether this is center to center or center to top?

From bikesdirect..........................

NOTE: Sized center to center, most riders fit a size smaller than they fit in a center to top sized bike
Example - if you ride a 58cm center to top sized bike then you may prefer the 55cm Kilo TT
Please refer to the sizing charts before making your own purchase decision

Sizing: (c-c = center to center seat tube, c-t = center to top, TT = top tube c-c)
44cm (c-c) is 47cm(c-t) with TT of 503mm* and standover of 28.5 inches*
47cm (c-c) is 50cm(c-t) with TT of 513mm and standover of 29 inches
50cm (c-c) is 53cm(c-t) with TT of 523mm and standover of 30.4 inches
53cm (c-c) is 56cm(c-t) with TT of 548mm and standover of 31.5 inches
55cm (c-c) is 58cm(c-t) with TT of 560mm and standover of 32 inches
57cm (c-c) is 60cm(c-t) with TT of 575mm and standover of 33 inches
60cm (c-c) is 63cm(c-t) with TT of 606mm and standover of 34 inches
63cm (c-c) is 66cm(c-t) with TT of 630mm and standover of 35.5 inches
(*estimated measurements of new size)