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Start with the original raw record
Benchmark mean is 0%, asset mean 1.2%, slope 2.1 and intercept 1.2%. The original hypothetical returns are not current market or risk-free observations.
Subtract the stated same-period rate
Let the risk-free return be a fixed 0.1% in every period. Benchmark excess mean becomes −0.1% and asset excess mean becomes 1.1%. Centered deviations and slope remain unchanged by this constant translation.
Recover the excess-return intercept
The new intercept is 1.1%−2.1×(−0.1%)=1.31%. Equivalently it is a+(b−1)f=1.2%+1.1×0.1%. The raw and excess coefficients refer to different response/explanatory definitions.
Retain the financial model interpretation
The tool fits whichever compatible return basis is supplied. It does not automatically subtract risk-free returns or certify a Jensen-alpha/skill conclusion. With a varying risk-free series, construct matched excess observations and refit rather than applying this constant-rate shortcut.