Possible easier solution: Wire the voltmeter direct to the starter "hot" post so it reads constantly, even when the engine is shut down.
Many (most?) voltmeters draw only a few milliamps of current - less than the internal leakage current of the battery and thus can be connected all the time.
Using an accurate VOM, measure the resistance across your voltmeter terminals. If it's more than about 2,000 ohms, you can safely leave the voltmeter connected constantly for MONTHS without appreciably draining the battery.
This is how the voltmeter is wired on my old Kenworth 10 wheeler. It can sit unstarted 6 months yet will start right up when I need it.