For frames with a round seattube, we know the toptube length is measured to the center of the seattube. Now in the case of the 595 seattube, which is oval rather than round, is it still measured to the center too?

I am asking because the seattube at the BB is round, and about a foot higher, there is a slight fairing (think original Madone) so that the seattube expands into an oval shaped shaft, so the center line of the seattube at a higher spot is different from the center line near the BB area.

In the attached, which one is the right center line, A or B?

For frames with a round seattube, we know the toptube length is measured to the center of the seattube. Now in the case of the 595 seattube, which is oval rather than round, is it still measured to the center too?

I am asking because the seattube at the BB is round, and about a foot higher, there is a slight fairing (think original Madone) so that the seattube expands into an oval shaped shaft, so the center line of the seattube at a higher spot is different from the center line near the BB area.

In the attached, which one is the right center line, A or B?

A is the correct line.