High pass filter attenuating frequencies equal to cutoff frequency
A simple high pass RC filter gradually reduces the amplitude of an applies sine wave as it falls in frequency. At the cut-off frequency the amplitude is already reduced by 3 dB. 3 dB is called the half power point and, in voltage terms, the output voltage will have reduced to 70.71%: -



It sounds to me like you should be aiming for a cut-off frequency that only attenuates at 30 kHz a fraction of a decibel. Maybe try 10 kHz and plug it into the fomula above or simulate it.

Alternatively you could use a 2nd order high-pass filter based on passive components that include an inductor like this: -



The beauty about this circuit is that you can vary "R" and get a peaking response like this: -



If you "tune" R to give a \$\zeta\$ value of 0.7071 you will get the so-called Maximally flat butterworth response. You would still choose a cut-off frequency that is lower than 30 kHz but you would get a flatter response down to 30 kHz and a larger attenuation of frequencies below 30 kHz.

The okawa site you looked at before covers this type of filter.