Gordon Shumway writes:

On Sun, 07 Sep 2008 13:06:21 -0400, Richard Evans
wrote:

Even if you don't have exactly 45, the two
cuts will be complementary and join perfectly.

Incorrect. The two surfaces may meet but the resulting angle of the
two boards will not be 90 degrees unless the cut is exactly 45
degrees. The OP implied he needs the resulting angle to be 90
degrees.

G.S.

If the outer edge of the V-shaped guide is 90, then it will be 90 even
if the miter isn't exactly 45. (e.g. 45 1/2 and 44 1/2.)

Both the interior and exterior edges may not line up (picture a 30/60
combined angle).