I would have been upset - and I think you were reasonable in pointing it out. People make mistakes. If they are unintended and it's corrected, no big deal.
Sounds like you did it in a pleasant way. After the clerk was condescending, I would have progressed from upset to angry.
Then I would try talking to the owner - maybe you could pretend you didn't know the person was the owner's son?
And if they were still jerks about it, I would take my business else where.
They could either put up signs saying they have minimum cuts - which I think would be counter-productive for them to do -
Or they could put up charts with the decimal equivalents and teach the clerks how to use a calculator.
I think the store (person) was wrong wrong and wrong in this case.
JoAnns and WalMart have these charts and sell fabric by the inch. In fact, one of the LQS here sells fabric by the inch.
Let us know what happens if you choose to pursue it.