no Dan i have both a radius probe and a flat one. my (stupidly anal) concern was calibrating the radius probe on a flat surface. like i said in reality the probe must first be calibrated to the mic using the flat disc standard on the bottom anyhow. then once the probe is calibrated to the mic you calibrate it to a standard preferably of the composition of what you're working with. in this case i have a couple pieces of chrysler block that i milled to within .001" flat. truthfully i've been able to hold the total observed error to less than .005" and closer to .003" so with regards to wall thickness i admit i'm just being stupid. in all honesty i don't think it's an issue i think i'm just OCD