We had to do some custom plate washer's a couple years ago. As I recall,
our conclusion was there is no really good rule-of-thumb or correlation.
IIRC, I still have the MathCAD sheet I used to find the thickness. I'll
look through my archives tonight. As I recall, you use MathCAD,
correct? If so, I'll send you the sheet as long as you promise not to
publish all of my mistakes. :)