I was on holiday in sitting in a cafe in Cheddar Gorge a few years ago watching a chap installing an alarm system in the building opposite. He used a measuring tape to work out where the bell would come out, he drilled the hole and still came out in wrong place right in the centre of a molding.
Ive always drilled by eye using common sense and judgement and normally the hole is where I predict it will be, I did a video doorbell for a neighbour once, I was a bit tipsy at the time (Sunday afternoon half a bottle of red), every hole drilled was perfect. He still talks about how I installed his video doorbell drunk