When I saw Michael Waddell speaking a couple weeks ago, while he was trying to sell everyone his new rangefinder, he told an interesting story. Many years ago he was hunting with an "old pro." When they got to the stand, the man pulled out a rangefinder and a calculator. He would range the ground ahead of him, then start punching in numbers on the calculator.The Pythagorean Theorem is exactly what he was doing.

You can do it that way. Though it's not neccessary.