"How do Americans measure potatoes?"
In bulk, it's pounds. Last time I bought bulk spuds in England, they were in a large bag that ever so logically was labeled "hundredweight", which equally logically is 112 pounds (or 8 stone, of course, which everybody knows).
"I can't help suspecting that people who were educated before metrication may be more adept at mental arithmetic in consequence of having to perform calculations in a variety of imperial units."
Direct observation suggests you are correct.