Steve,
I see you have not received an answer on your question yet - I'll give it a try although our kitfox is an S7.
The method in the build manual is just fine to use for setting the dihedral.
I bought a 4' level which was useful for many other things, and, it served the purpose of setting the rigging of the wings, leveling the fuselage, etc.
The 7 has less dihedral than the 3 but just use the spec - the procedure is the same. Go ahead and use the method in the manual. easy-peasy