Ditto what everyone else said and also,
Judge the pressure of a clamp head radiating (fanning out)
on a line of 45 degrees each side of the clamp head. This
90 degree (2 X 45) arc is the area the clamp head is
exerting pressure to/on. By pulling the clamp head away
from what's being clamped the width of this "pressure area"
is widened. With a caul a wider area can be clamped using
The above is also true when gluing up boards into panels.
Let's say you are gluing up two boards 5" wide. The first
clamp can be positioned 5" (the width of your board) in from
the end. The remaining clamps can be position 10" (twice
the width of your boards) on center with the last one 5"
from the end. Any more than that is overkill and of course
it never works out to be exact but the idea is to be as
close as possible without beating yourself up over it.
This is not to say that if your stock is good and true you
cannot get by with less, just that if you are ever
wondering, this is a good rule to go by.
Your Mileage May Vary and Bingo Was His Name-O.
Oh, and before I forget, cauls are tools. It doesn't hurt
to have a few on hand. They can be made from left over
scraps and stored away until needed.