Center to top of seat tube is a legitimate way to compare within the same brand of cycles or even type but since many manufactures leave the seat tube as much as an inch above the top tube or even more this can prove frustrating when trying to compare bicycles. The effective top tube length c/c is the best way, I suppose but failing that, measuring c/c of the seat tube is useful.

It does not mater what Centurion says the size is, what matters is how it fits the OP and how it compares to other bicycles he has in his/her fleet or experience.

J