how perfect are your boxes?

That "forumla" is special.  It has a name!   :)

http://en.wikipedia.org/wiki/Pythagorean_theorem

I love math and am good at it.  However, when I'm building a box for work, I'm not spending the extra time to calculate the angles that I need.  Here's a few tricks that I follow, that are basic and anyone can do them.

One thing I'll do is cut my side peices first that have the angles on them.  For example, if I'm building a truck box with an angled front, I'll cut the sides out first.  I use the depth measurements for the top and bottom and cut accordingly.  If the truck I am cutting for has a top depth of 4 inches and bottom depth of 8 inches, then i'll cut my sides with a top dimension of 2 1/2" and bottom of 6 1/2".  That allows me 1 1/2" inches for two thicknesses of 3/4" MDF.  Now, the overall depth of the box will be reduced by less than 1/32" which I can live with.  Using the cut out side pieces, I can now measure the angle at which to cut the front, top, and bottom pieces and tilt my table saw accordingly.  To cut to the specific lengths, it's cut and fit.  I'll cut the piece, then fit, and if it's a bit long, trim it up, refit, etc. etc.  I do this escpecially for the front since there's two angled cuts. 

Another trick I'll do, is actually draw the side profile out in real scale.  I can then take my measurements and cut away. 

I always cut out my side pieces first becuase all the cuts are at 90 degrees.  Then I'll use that as the base for fitting all my angled pieces.