Kumpol do you have plans you're working from? Those should specify whether your stem mold edge A gets secured at C or if it's edge B that's of concern.

My guess is it's edge A should be 18" from the nearest mold frame. Putting B at that point would severly alter the curve of the strips being formed to shape for the hull.

Lacking actual instructions in the plan you can always take a few strips and tack them in place on the molds, see what kind of shape they take on. It'll be quickly apparent if stem mold placed with edge B at point C makes strips curve too abruptly as they pass over and beyond that closest frame just to the stern. 

   SP makes a good point. Another approach would be to measure overall length of boat and make sure overall length is 16', or something close. The form position might be an inch or so short but you'll make it up with the thickness of the stems.

   spclark and Silver Salt, Thank you so much for your kindness advice