Looking into making my own climbing holds for an indoor climbing wall at home. From what I understand they have been made. I just need to figure out the shrinkage of my clay so that the mounting hole is a standard 3/8" like commercially made resin holds. I vaguely understand the concept of shrinkage calculation. Roll a slab the same thickness as your finished product, mark with a ruler 10cm, fire at temp and then check against the original ruler for % shrinkage. Do I just increase the desired hole size by the % and voila? Or is there something else to it? Thanks for any advice.