Steem Power and Steem Rewards are not valued until the time of exercise.

This, I assume, you are meaning at the time of Power Down - in the case of receiving a 100% Powerup of rewards...

So essentially, you’d need a method of tracking every single rewards payout in Steem Power - and then a protocol to determine what fraction of that are “exercised” as a Power Down, each week after for the next 3 months (if your Power Down is on the 3-month setting, versus older ones like my account got initiated on a 2-year breakdown) - adding up those weekly fractions for all 13 weeks, get the actual figure to be calculated as “income?”