I don't know how others do it, but this is what works for me. I make a trigger template out of thin card stock. I fill the sear hole with some "silly putty", you could probably use another putty of sorts, but this stuff works pretty good as it is easy to remove when done. I then insert the lock into position. Remove the lock and the putty leaves a nice impression as to where the sear and sear arm are located. I then lay my template on the side of lock panel and pencil in the shape to engage the sear. Cut the template and make sure all is good, transfer that to the actual trigger and shape. I'll use this to locate the pivot hole as well. I put the hole on the same plane as the sear arm (which you will see with the impression), about 1/4 to 5/16 from the contact point of the sear. If you want a harder pull, you can move this pivot location a bit further away. Also, err on the larger side when cutting the trigger height, then when installing you can file to final dimension(or sneak up on the final dimension) to eliminate the freeplay.
