To use your deck as an example, rest one end of a board on a joist hanger attached to the ledger. Mark the top edge of the board 6 feet from the end resting on the hanger. Next mark the ledger 8 feet from the board. Drive a nail into that mark, hook a tape measure on the nail, and extend it to the mark on the joist. Now move the board and the tape from side to side until the 10-foot mark on the tape lines up with the 6-foot mark on the board. The board will then be perfectly square to the ledger. You can drop a plumb bob anywhere along the board to pinpoint the spot to dig your footings.