Position sizing based on Balance or Equity

It would be great to have an option to select a position size (volume to trade) based on the Balance or Equity.
There could be two radio buttons in the Market Watch and Chart QuickTrade panels with which the user could select whether he wants to select from the list of absolute values (or enter an arbitrary volume); or he rather wants the position size to be a fraction/percentage of the Balance or Equity.
The % value should be chosen from a dropdown (or any arbitrary amount could be typed in), and that whether Balance or Equity should the calculation based on could be set somewhere in the application’s settings.