Scratcher, I am a professional aero engineer. There is no master formula, I do wish there were. We never started from scratch with a new design. If I were going to scratch build a nice pattern ship I would get a photo, or better, a drawing of Chip Hyde's Hydeaway airplane and copy it . You can cange the looks a little by canopy shapes, different nose cowls but it would be guaranteed to fly. It's the way most designers work if they would admit it. It is a form of evolution by cross breeding. If I were after a trainer I would copy a well known and regarded trainer. Same with gliders, racers, etc.

If you want formulas of your own. Measure the Hydeaway or the trainer and develope your own formulas. You will have horizontal and vertical tail areas as a percent of wing area, fuselage side area distribution, and the like. Wing and horizontal tail positions can be determined by careful observation of what works. There is nothing like competition success to prove the worth of aero formulas. It is much better than a years work with computers and formulas can produce. Given a good flying design, the knowledge of the aero formulas, etc. will help optimize your work but copying is the single best method of assuring success.

Now, there is a lot of fun in knowing why things work, certainly that is the reason I went into aero eng to start with. The last one even points out the interaction of experiment and test and design.

http://www.desktopaero.com/adw/welcome.html

http://142.26.194.131/aerodynamics1/

http://www.av8n.com/how/htm/intro.html