Although I'm pretty sure you can not loose Steem Power when upvoting things, your idea nevertheless is pretty good. (I guess you meen the Voting Power or Ressource credits which are going down).
For those two thingw I'm going to implement a slide controller with which you can control the minimum percentage you want to have left. It's going to come in the next few weeks!
Thanks for your suggestion and feel free to always share your ideas :)