Just be sure to distinguish between nominal size and actual seat tube length. Nominal size may be actual seat tube length, effective seat tube length, or nothing real at all, just a number out of the designer's head. Same for top tube. Example: a Giant M (medium) has a listed seat tube length of about 50 cm (I forget exactly, but that is close), but that size Giant is essentially a perfect 54 cm bike in old time horizontal top tube terms. If you look on Giant's geometry chart, and see 50 cm, man that can be confusing. Make sure you know what you are looking at. The best approach is to speak only in terms of effective tube lengths, i.e. think as if all bikes had horizontal top tubes. Just establish what the manufacturer means by its nominal bike sizes, then establish which of its measurements are the effective tube lengths. I hope this is more help than hindrance. This stuff does get to be confusing.