The middle note should be set to approximately the center of your midi keyboard. This is especially important when using scales with other than 12 notes. Playing the middle note will always result in the same pitch, no matter which scale is loaded.
The middle voltage is typically set to the same note number and the pitch then adjusted via the pitch knob of your vco. You can set the middle voltage to other values if required, but note that it might cap your pitch range on the lower or upper end.