Best bet, for the most bang, would be to just use 2+ bottles.....
Since most gas gauges are "inherently" incorrect......now wait, hold on, I don't mean they are lieing exactly lol.....but I read somewhere the manufacturers actually make the gas gauge read a "full" tank longer, than a half tank......
Sounds confusing, but have you ever noticed, how once you pass that half way mark, your gas gauge just seems to "disappear" to E? Yep.....that's why......it's sorta supposed to stop people from riding to E......or something.....and to make them not think there car is using so much gas

I think I read this on car talk or something......
Now granted, there are times when carbon/gum will develop on the fuel sensor/sender.....and cause the gauge to misread.....THIS scenario is not related at all......that's a totally different scenario
The only thing close I could find, was this wikianswers post:
http://wiki.answers.com/Q/Does_a_car_use_more_gas_when_the_tank_is_below_half_full
But yea......sadly, no cited sources lol....as with all wikis, you gotta take it with a grain of salt.....BUT YES.....here's the 2 I seen fit most:
Quote:
It also happens to be true that some manufacturers install gauges that read higher than the actual percentage of gas left. This is because people like to see their tanks "full".
Quote:
It may seem that it takes longer to go from full to 1/2 than it does to go from 1/2 to empty. That is because the fuel monitor inside your gas tank does not go all the way to empty. It's range is Full to about 1/8th. So, full is full, and empty is really 1/8th. It is designed that way so you won't run out of fuel. But, dont run it that way. Fill back up when you get down to a 1/4 tank because the bottom of your tank has trash and other elements that can hurt your engine and clog your fuel filter.
Quote:
Your gas gauge, like almost every other car's, just isn't a precision instrument like an airplane's would be. If you have a 12 gallon tank, you can probably put in at least 4 gallons when it say 3/4, 7 or 8 when it says 1/2. The first and last quarters always seem to hold more than the two "middle" quarters.
But yes.....I'd recommend 2 bottles.....simply because, if you "guesstimate" 15 gallons - in a partial tank of fuel......how low do you really want to "take" that tank, that is already, more than likely less than half full
So yes, you'd get the best cleaning affect by treating with 2 bottles
