Here's another version of a hole-saw guide for drilling lockset holes in a door stile. I think it is simpler and more versatile than other versions I've come across because the screw that holds it to the edge of the door also locates the center of the latch hole.
As shown in the drawing, I made the guide out of 1x pine. I bored 2-1/8-in. dia. holes in both sides of the guide for a 2-3/8-in. back set. I use a hole saw to make the 2-1/8-in. dia. cuts, chewing through most of the door from one side and then finishing the hole from the other side to prevent tearout. I back out the holding screw, and I'm ready to drill the latch hole.
I made up a variety of these guides, so I'm ready for doors that are 1-3/8 in. or 1-3/4 in. thick, and locksets that require either a 2-3/8-in. or a 2-3/4-in. back set. They are equally useful for new or old doors. And because the holes in the sides of the guide index the position of the saw, I don't need a pilot bit to guide the hole saw -- ideal for retrofit work.
WilI Hesch, Atascadero, CA