To shorten a branch

We usually cut back more than 15" to reduce a limb by 15" -- because we are cutting back to a side branch which itself has some length.

For the best results -- even growth and less chance of multiple shoots or "witch's broom" emerging -- cut to just above/beyond a side branch that is at leat 1/3 the diameter of the branch being cut.

Here, arrow "A" points to a short side branch (we've drawn a red line parallel to it, to help you see it among all the other twigs). That will be the limb's new tip. Arrow "B" indicates about 20" of branch being removed in the cut-back.