capacitor in series,is that Butterworth?
A single-capacitor filter, with an R, has too few degrees-of-freedom in the pulse response; basically, you have NO ADJUSTMENTS POSSIBLE. Only the F3dB is adjustable. Overshoot does not occur, so cannot be adjusted.

EDIT The shape of the magnitude response of lowpass RC filter can only be

$$1 / sqrt[ 1 + (Freq/Freq_3dB)^2]$$

Here are mag/phase for 0.1 to 10Hz, 10Hz to 1KHz, 1KHz to 100KHz. Notice the phase shows a useful change per decade.


Above F3dB, that same deltaPhase occurs, as phaseshift approaches 90 degrees.