-cvddc -100 - Apparently you have to set it to relative values.
I use absolute value for all of my 5700XT cards and 6600. Is this special for 6600xt? The API help says "Set GPU core voltage in mV (0 for default)" and doesn't mention the relative value. May it works also.
Yes it is probably for all 6000 cards. It is mentioned in either the readme.txt or the releasenotes.txt somewhere at the beginning. I also tried the absolute settings first but PM would nnot pick it up so I started to read the readme and releaenotes a bit better, what I should have done first

If you don't use a relative value it will say so in the phoenixminer terminal window. When you set it a restart is needed as it cannot do it on the fly like with some other settings.