I don't suggest using the 'three fingers behind the first spar' method. That could work on some planes, but if you do that on all of your planes you'll miss more times than not. Many planes have the first spar on or very near the balance point and three fingers width behind that could be a disaster on certain setups.

The most surefire method is to calculate your mean aerodynamic chord, or MAC and balance as a percent of that. Most conventional planes (one lifting surface, one horizontal stabilizer behind the wing) will be happy around 28-30% MAC. This works well for straight wings, tapered wings, and wings with either forward or rearward sweep. Deltas like to be somewhere between 15 and 20% MAC, and most flying wings like it around 12-15%. Calculating bipes is slightly tougher, but you can use the same method, but calculate the MAC from the planview of both wings.

A few things to keep in mind:

-The lower the aspect ratio and the larger the horizontal stabilizer, the more room for error you'll have.
-Any conventional layout designs with a lifting surface at the location of the horizontal stabilizer will want to have the CG farther back, thus making the H-Stab a true lifting surface. These are rare, but you may run into a few of these types in old timers or free flight.
-Canards are a bit different, and I've yet to come across any good and simple method of balance point estimation that works every time.
-If in doubt about your calculations, build a smaller chuck glider of similar demensions to test your work.