Is there a rule of thumb for aircraft design? I'm wondering how to best scratch build an rc aircraft and what rough proportions the aircraft should be in.

I'm aware of the fineness ratio that determines the width to length ratio of a cross section of a wing should be around 1:3 or 1:4, but I can't seem to find similar rules for:


The distance between the wing and tailplane
The length of the wings


Am I just better off just making a scale model of an established aircraft instead of using these rules?
No, there isn't any single rule of thumb for airframe shape.  Just look at the huge variety of airframes that have been built and flown over the years.

One way to scratch-build is to start with a cardboard glider that's roughly the shape you want.  Tape the wings and tail to the fuselage in different places, tape on some noseweight, give it a toss, deflect the elevator, give it another toss, vary the dihedral, sweep, decalage.