Okay so I did your measuring trick and came up with 11 3/4 so I'd be 29.845
That makes things pretty easy.
As you know rails to groomers are pretty far apart in terms of board design. We typically advise a do it all design for riders who want one board to cover park, rails, groomers, carves. If you wanted to focus on a park and rail board that would be a different suggestion. Clarify that for me and we will proceed.