So you say you keep an x amount of dollars always in your wallet, so your spending money increases when you sell? Not too smart I would say, because your money doesn't grow.
For my spending wallet say I start with one coin in there, @ $600. When the value in USD goes up say $750, I'll spend $150, never going below a certain $ amount. Of course this only works as long as it rises, if it no rise, you no spend. If it keeps rising, eventually you'll spend it all, but that coin was designated for spending anyway, you just got a hell of a lot more mileage out of it, and supported the bitcoin economy with it.
This way you can safely take profits knowing there's a limit and a brake on it, and you won't end up eating through all your stash becasue you have held coins in reserve for the super big run ups, if that happens.