96 speedster reupholstery patterning problem

[hence]

We are reupholstering our 96 speedster with marine vinyl from joanns. So far the center seat cushion looks good but we are about to do the bench part of the seat and I have attempted the back seat and have failed to sew something that fits good twice now. Does anyone have any tips on how measure how much fabric to put between the 2 seams on the bench seat and any tips on how to pattern the back seat. I should have just saved more money and bought vinyl from seadooupholstery.com but its to late now. Sailrite has been a great resource for redoing the seats but I still cant quite get the size right for the back of the seat and Im really frustrated and tired of wasting vinyl. The first time I made it to small because I tried to pattern the boxing after the seat cover but after I watched the sailrite videos the boxing needs to be a straight piece of vinyl and so I did that and added an inch to the bottom and then it was to much. Any help would be greatly appriciated.

 

[Thewetterthebetter]

Cut the old upholstery stitching apart and use the pieces as a stencil to trace the new ones or at least get you the measurements. It's tedious but that's how my father did it on his bayliner