Yea, it's gonna be fluctuation in value over the powerdown. Would be great if hitting the power down button set it in stone hehe :P Catch it on a week when steem value is high and you'll get a good payout, then cancel the power down when it goes down in value. I'm pretty long term personally so I'd say wait til steemit really takes grip before doing a full proper week by week power down. Could be worth an absolute fortune in the next few years.