Anybody knows what is a safe voltage (--bitburner-voltage option)?

These options give about 22 GH/s per 3 miners:

cgminer-nogpu.exe --avalon-options 115200:6:10:32:400 --queue 4

I tried --avalon-options 115200:6:10:28.8:440 but this was slower. I did not try overvolting.