assumptions--plane needs to fly a straight, unmixed knife edge; stab trails the wing and is actually in the plane's tail.
depends on several factors: wing needs to have correct dihedral and placement vertically such that pitching isn't confounded by rolling upon rudder application. Downthrust--just enough to keep a straight power-on vertical up line (pitching to canopy or belly). Fuse shape--large, bulbous fuse topside and flat bottom side in general don't require a subfin on rudder and often require a dorsal on top.

What you should be trying to do is to raise center of pressure on the fin-rudder surfaces. Lower stab than thrustline helps with this type fuse because it exposes more fin-rudder above the stab. Increasing fin span would help (also increases rudder span and less below and more above stab). Often an aerodynamic balance on rudder helps the bulbous topside fuses.

For this fuse type the tail should fly sl. high in S&L. That requires either positive inc. in stab (about 1/2 deg is enough usually)

If your fuse was more balanced laterally in terms of area distribution, then rudder-fin area could be more evenly distributed about the stab. This fuse type often could use some subfin in tail.

There's alot to it and it's hard to get it exact the first time without cutting and regluing. Something you may wantto play with is small chuck gliders built to your correct scale planform, in which you change the relationships of the surfaces. There's alot more of course, but this will get you going.

MattK