I think you identified the parts correctly. Considering the messy equation, have you contracted to cable capacitance and the input capacitance of the oscilloscope into a single value? This might simplify your calculation a bit (cable and input capacitor are in parallel).

However, you will get a fraction with \$j\omega\$ as well as some non-frequency dependent terms in the numerator and denominator. This is to be expected. Your goal is to select the capacitor, so both terms are the same (apart from a real valued factor), so the result becomes independent of frequency.

I think you identified the parts correctly. Considering the messy equation, have you contracted the cable capacitance and the input capacitance of the oscilloscope into a single value? This might simplify your calculation a bit (cable and input capacitor are in parallel).

However, you will get a fraction with \$j\omega\$ as well as some non-frequency dependent terms in the numerator and denominator. This is to be expected. Your goal is to select the capacitor, so both terms are the same (apart from a real valued factor), so the result becomes independent of frequency.