If you have access to an oscilloscope, you can monitor the input and one of the "raw" reverb outputs, and adjust the GAIN trimmer P101so that they match. Note that the reverb output will probably have greater excursions than the input signal, especially if you are just using a basic wave shape from a VCO. As such you are setting the level of the "bulk" of the waveform. The alternative method is to adjust the GAIN trimmer until they sound about the same.

The feedback trimmer P103 is adjusted with the [FEEDBACK] pot set to maximum. Adjust it to taste.