No sense in worrying about voltage. You have no way of knowing when the knock window is or what frequencies to look for.
As dream3r said, just log retard. Most people log dwkrz though because that is the retard actually applied to the output ignition angle.
Doh! I was logging wkr+i in my Volvo.
Will need to read through %KKRA again, zwbas = zwgru + dwkrz(i) + wkrdy is what you are referring too? Teach me for skimming the FR!