[R_1 = \frac12.2 \cdot f \cdot C_1]

[f = \frac12 \cdot R_1 \cdot C_1 \cdot \ln(3)]

[R_1 \cdot C_1 = \frac12.2 \cdot f]

Or more simply for design purposes:

[f \approx \frac12.2 \cdot R_1 \cdot C_1]