Great question Samsrc, Usually you will be attaching your floats to an existing wheel bracket on the plane. What I mean is that your forward gear on your taildragger planes and your rear main gear on your trike planes usually becomes one of your mounts. Then you make the other mount accordingly. For high wing trainers (that have a flat bottom fuse), I like to just make another main gear identical to the rear main gear and mount it right behind the cowl. So to answer your question, the set distance apart that your gear already is, becomes the span between the floats.

All the set up figures you mentioned are true. Make sure that your float tips end up at least 2 inches in front of your propeller or more. Also, the incidence of the wing matching the incidence of the floats is true only for flat bottom wing trainers. If you have a symmetrical wing then you want to have positive incidence in your floats. For my low wing Cloud Dancer 60, I used 3 degrees and it worked great. What that means is that the nose of the plane is further away from the floats than the tail when looking at the plane from the side. It looks kind of funny, but my Cloud Dancer worked great and I'm glad I had the positive incidence as the water handling of the plane was superb.