It really would depend what your sett is. I just wove a sample at 30 epi for a project, the draw-in was about 10% and shrinkage roughly another 10%. However, mine had a heavier linen creating stripes in the warp. You could weave a sample at the beginning of your warp.. If you measured it on the loom, cut it off, hemmed it and it was as square as you wanted it to be, you’d know exactly how much you needed to weave for each napkin.