Yes, thank you, Bill. I skimmed your link and found several references that might have led people to believe that "rule of thumb" means what I posted above - but the woodworking theory that Angel mentions is far more likely.

Artists also measure things in a rule-of-thumb kinda way, though not necessarily with their thumbs: they are taught to hold up their drawing implement "against" the part they are trying to draw, and then drop this "measurement" onto the page in front of them.

Perhaps that's "rule of eye"? or "rule of pencil/charcoal/conte"?!

Let us go in peace to love and serve the board.