When laying out a rough opening of, say, 7 ft. 9-5/8 in., in the center of a room that is, for example, 15 ft. 7-11/16 in. wide, try this: Jamb your tape to one wall (it doesn’t matter which one), and put a mark on the subfloor at the 7 ft. 9-5/8-in. mark. Next, measure what’s left on the other side of the mark and halve it. That halfway mark is one side of your rough opening. Measure over 7 ft. 9-5/8 in., and you’ve got the other side.
What you’ve done is halved the halving, thereby halving the chances for a division error. You have also eliminated one movement of the tape and made your rough opening in one measurement rather than the sum of two smaller ones.
Jim Chestnut, Pound Ridge, NY