We use the same formula that eosio uses to pay producers for vote pay. We calculate the increase in vote pay that the BP receives from the proxy and send back the percentage set by the proxy to voters.
A BP has 50,000,000 EOS votes which yields 102.5 EOS daily in vote pay. Let’s assume the rewards proxy have 12,000,000 EOS assigned to the proxy. If the rewards proxy voted for the BP then it would increase its vote weight to 62,000,000 EOS, which would yield around 126.5 EOS daily.
From this, the rewards proxy vote increased the BPs daily yield by 24 EOS. Therefore, 80% of this added yield will be returned to voters as a reward — 19.2 EOS