Haven't had any feedback; does it matter if the thwart is fore or aft by a half inch or so?

The distance between the bow and the bulkhead should be shown in your plans. If the dimension isn't explicitly listed, you can scale it from the drawing. A half inch off either way isn't going to noticeably change the helm balance, if that's your concern. But if you're still nervous, I would contact CLC.

Old Yeller 

BARRY,

I would suggest using a plumb line from the middle of the hole in the mast step, you want the thwart to be centered over the hole, so the mast is straight.  It should not make a difference it the measurement is exact as long as the holes for the mast partner and the mast step line up.

Skip

Yes, getting the mast rake right is very important. Again, not being familiar with the plans, it very well might be raked slightly aft, rather than exactly plumb to the waterline. The plans should make it clear how to arrange the mast step and partner (hole in the thwart).

Old Yeller