That 75-80% of fuselage length often works.
But a better method is to measure from the prop to the aircraft's CG and multiply by 2.5 then place the centre of the float at the CG and the step just behind. This method guarantees that 10% of the float will be ahead of the prop which is important.

For details see [link=http://www.gvmac.com]gvmac[/link]
Go to Articles and read or download the Word doc. The first couple of pages tell you how to size and shape floats, then goes on to how to make them yourself.

Quote Hangar 9 Floats SPECIFICATIONS
Length: 36 in (914mm)
So you think the Hangar 9 floats should hold up 9-10 lbs ???
If each float is about 4" wide and 3.5" deep, that sounds about right. They are good for up to a 10 lb model.