I'm not sure why you wouldn't use the measuring device built into the bottle?
You obviously trust the science that was used to develop StarSan, why would you potentially mess with the ratio? You said, you mix a little less than 1/2 an ounce and fill the bucket with about 2-2.5 gallons... Any idea at what ratio it becomes ineffective? Or at what ratio it is no longer no-rinse? I don't know the answer to either question but assume, the guys that developed the product do... and they put the correct ratio on the bottle.
You only get one chance at santitization... If you find later you have a problem, its too late your beer is ruined. This stuff is not like adding chilli powder, until you get the taste you want.
I guess it'd be different if the measuring tool wasn't built into the container.
Sorry... Rant over...