I do not remember exactly, but I am sure it is measured from touching the sheet to the rollers and as far as I remember (but I'm not sure if 3cm from the sensor tip selection, you need to check it) ~ 5mm from the edge. Make a hole of 0.5mm and if you do not hit it you can enlarge it to 0.8 or 1mm.
I will write how it works so that you know where to look for a possible error. You have three sensors in the printer, named in the tutorial mfps, pis, es and npis is an additional, because these original three sensors are removed from the printer. All three sensors must go through the specified time for all three sensors. The most important is pis. When the uC board starts, it switches mfps programmatically, after ~ 500-550ms (time between the sheet metal position on the drum and the hole position in the sheet on the photoresist), when the hole in the sheet is on the photoresist (npis), the UC switches on, counting from the writing ~ ms delay and printing starts. In other words, the most important thing is to write, since it starts printing, mfps and es are actually paper jam sensors.