Tip on Rabbet cut setup for a newbie?

OK, I'm making the "Limbert Tabourett" table from Popular Woodowrkinga few months back.
The plan requires a 3/8" rabbet in the coner of each legs. Using a good table saw with a dado blade and an auxillary fence. I cut three of them, and I think I screwed one of them by cutting it too big.
I just used a square and test cuts to make the measurement for the fence and blade. It occurs to me that there may be a better way. Any tips on setting this up to ehlp my chances of getting an exact cut?
Many thanks for ideas
Jim Helfer Pittsburgh PA
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