I had a problem with another homebuilt and needed to figure out whether it was the idiot light or the voltage regulator. I rigged up a temporary digital voltmeter and went flying. I was looking for around 12.3vdc with alt switch off. And around 14.4vdc with the alternator on. Turned out the idiot light was correctly indicating an overvoltage condition with 15.5vdc on the battery bus under high RPMs. And, I had a failing voltage regulator.
Hope this gives you some ideas.
Mount Vernon, WA
2019 VAF Supporter