Do you have  a reference? I haven't found anything about how power is distributed on a card from
the different sources. I just use simple math, divide the total by the number of connectors, with
the 8 pin connectors counting as 2.

Edit: That 20W figure looks like BS. Someone on reddit calculated it for a 3080 assuming the 2x8 pin
connectiors would draw their full 300W of the total 320W for the card, leaving 20W for the slot to supply.