Forget measuring to locate recessed cans when you’re hanging drywall; Blind Mark’s Center Mark tool uses the awesome power of magnets to make finding them faster and easier.
To use the Center Mark, screw the target into the socket inside the recessed can and hang the drywall. Then move the Center Mark locater puck approximately where the can should be, and with any luck the puck will snap into place to locate the center of the can. Finally, saw away from the puck until you hit the edge of the recessed can, jump to the outside, and saw around the outside of the can.
The Center Mark sells for $18. The Home Depot site claims the tool is only available online, but my local Home Depot had them on the shelf.