RobTi, good evening.
One way I have used that is generally reliable is.
1/. Mark the position of the lock bolt on the door frame edge.
2/. using a square, transfer the upper and lower marks from the lock bolt on to the face of the door frame.
3/. Close the door so it is firmly closed, using a thin strip of metal, slide the metal in between the door and the frame until the metal touches the lock bolt.
4/. I hold the metal strip hard, after it has touched the lock bolt, open the door, and holding the metal strip in the same position that it was in when the door was closed, mark the end of the metal strip.
You now have the upper and lower lock bolt positions nailed, you also know where the lock bolt will land when the door is closed.
5/. If you are unsure? next move is to remove a small amount of timber from within the lines marked above, just enough to allow the lock bolt to catch the slight indentation you have formed in the door frame.
You can then adjust the final position of the lock keeper from the indentation you have made.
6/. As for fitting the lock keeper itself, holding, or using the screws provided screw the keeper on to the frame, then using a Stanley knife run around the keeper so you deeply mark the door frame, remove the keeper and remove timber from the frame to the same depth as the keeper.
7/. Check the position of the lock bolt to ensure a positive rattle free contact between latch and keeper, if OK cut more material away from the frame to allow the lock bolt is not grounding out in the hole you have created.
Ken.