Should I use the BA II Plus percent key to convert every CFA rate into a decimal?

No. Treat the percent key as context-dependent, not as a universal conversion shortcut. In a displayed formula, it is usually safer to type the decimal directly. For example, use 0.075 when a formula requires r = 7.5% as a decimal rate.

In TVM worksheets, the context changes. The I/Y register expects a percent rate per period, so 7.5 is the natural entry for 7.5%, assuming the period matches the cash flows. The mistake is using the same input style everywhere.

A good exam habit is to write the format first: r = 0.075 for formula work, or I/Y = 7.5 for a TVM register. Then calculate.