Is there a rule of thumb for aircraft design?
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.