You want the CG on the forward end of the CG range to start with.

Put the step of the floats 1" to 1.5" aft of the CG.
That should leave you 4" to 6" of float ahead of the prop.

My rule of thumb is to make float length 2.5 times prop to CG distance. (Your 42" floats sound a bit short for a plane that size, but maybe it has a really short nose.)

That rule would make floats for my 100" span 26cc petrol powered sport/trainer 58" long.