Matthew, it appears to me that you are going about this the hard way…I would suggest that you drill a small (1/4″?) pilot hole through the entire apron so you have a reference on top and bottom of the apron. Drill the appropriately sized hole for holdfast from the top to about 3 inches depth (or another dimension that allows the holdfast to work). Then from the bottom drill an oversized hole until it meets the correctly sized one, in effect reducing the thickness of the apron as far as the holdfast is concerned. I have seen this recommended in several places to remedy the situation you are in. I have not noticed any deformation in my 3″ thick Douglas Fir top, but I have mostly converted to the PS “clamp in a vise method” and don’t use my holdfasts too much anymore.