I set it to zero. The way he's trying to fudge factor makes absolutely no sense. There's no reason for a whirlpool addition to contribute more bitterness than a 5 minute flameout addition. It should be treated as a flameout addition, with a multiplier at the very least.
With that said, we know basically nothing about how boil off rates, kettle geometry, boil pH, protein content, or varying temperatures impact isomerization. It's all guess work, and a cool temp <180F whirlpool addition is going to contribute minimal bitterness, if any.
Hey man, IBUS are just a construct. Stop trying to harsh my mellow.