Hadn't given it much thought, since I use flex, and don't have any perfectly specific radius. I did note that around my wye, things got a might skewed off center, and maybe this was the reason.....or, I laid the track out just off what the actual geometry might do.
I suppose you could lay to the right or left edge, depending, just covering the sharpie line. When it disappears, put the narrower side down on the other side.