Impedance matching for custom level translator board
Impedance matching is needed when terminating a transmission line. Almost any (wire, pcb) connection can be considered a transmission line when the length of the connection (wire, pcb trace) is longer than the wavelength of the signal.

Your signal is a logic signal of up to 20 MHz. Since that signal will have steep slopes (it is not a sine wave) it will contain higher order harmonics. Let's assume these harmonics are relevant up to 200 MHz, then we have to consider that our signal has a 200 MHz bandwidth. 200 MHz means a wavelength of:

\$\lambda = c/f = 3*10^8\ /\ 200*10^6 =\$ 1.5 m

Is your signal travelling over a connection in the order of 1.5 meter or more?

If yes then you need to use a cable with a certain characteristic impedance and terminate that cable at both sides with that impedance. Then the effect of signal reflections will be minimized and your signal should travel without being harmed much.

If no, your connection is much smaller than 1.5 meter, then you don't need to use a transmission line connection. You don't need to consider any characteristic impedance and you don't need to terminate the connections. Since the distance is quite short the effect of signal reflections should be negligible.

Note the relation between bandwidth of the signal and distance. For very high frequencies like the connection between a CPU and RAM running at a few GHz, even the small distance on a PCB needs a proper transmission line. The old analog phone system uses very low frequencies (a few kHz) but the connection still needs to be terminated properly as the distance can be very high (many km).