Formula,most balance tubes are located nearer the carb that the head, but I don't think the location is as important as as the tube itself.
Carb jetting is a trial and error affair,requires some knowledge of how the strength of the mixture affects driveability, and yes it's best to start with the stock jetting and go from there.For instance if you need to lean out the part throttle mixture, you change the main jets down a few sizes until the engine stumbles slightly on light acceleration, then go up a size.But many guys forget that also changes the full throttle mixture,so now you may have to increase the restriction size on the so called power valve which provides the necessary richer mixture for high load or wide open throttle.Generally, on a multi carb set up, if using carbs originally designed for a single carb intake, you will have to lean down the power valve since now you have 3 instead of one high load enrichening circuits.The Holley Weber was designed to enrichen a 120 cubic inch engine,3 of them on a 302 cube engine may work out fine with no jetting changes. You might want to get some carburetor books,Jeff Williams has some Holley manuals,you can pick them up used on Half Dot Com or buy them new.
I have a number of Holley spec books,listing the jet numbers and such. When you decide on a carb,email me and I can give you the info.
