Solidworks 2013

Hi everyone.   How does one go about constraining a scaled part?   I know that you can change the ratio down to what ever you want 2:1, 2.11.  But if you want to reach a final diameter for the scaled part for instance, can one constrain the scale or perhaps make it a driven dimension, versus driving?  The only option I've found is trial and error until you reach your target dimension.

Why would you want to do this?  There are times when you design a part for aesthetics and when you achieve just the right geometric ratios that make the part "pretty" you finally need to scale it to fit, or you need to build parts that mate to a larger/smaller version of that part.