I found a web site (Colorado Cyclist) that gave this formula for frames measured center to top C-T:
inseam * .67 = frame size C-T
I stood barefoot against a wall riding a book spine like it was a bicycle seat and had my partner mark the wall. 91 cm for me. His was 76 cm.
76 x .67 = 50.94 cm for him (51cm)
91 x .67 = 60.97 cm for me (61cm)
But the site says riders 6' and taller might find inseam - 27 or 28cm more comfortable for frames measured C-T. That would be 63cm or 64cm for me. The Surly LHT only goes to 62cm. Should I be looking for a larger frame?