Usury is confusing stuff. But as I understand it, the Credit Reform Act (1995) simplified the issue a bit. The general rule of thumb is 25% simple interest as a maximum on direct loans to any borrower using any collateral.
For the most part, the market won't allow a greater rate than that (at least, not without running into fair lending, Section 32, etc. issues). You can rely on other authorities for selecting your interest rate, but this will depend on circumstances - get with a lawyer if you're looking for a way to charge a higher rate.
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Opinions are Bartman's, not those of my employer. "A noble spirit embiggens the smallest man."