If you use the 3-2-1 method you can't go wrong. Three small drops on the slide (one drop on each rail and one on the forward part of the slide where the barrel lug rides), two drops on the barrel (one on the barrel itself rubbing it around the barrel to coat it. One on the barrel lug.) and one drop on the piece of metal at the rearmost part of the frame (senior moment. I don't remember what this part is called).
But, after all, this is a Glock. It will function if over lubed. The down side to over lubing is that the excess lube will tend to attract debris and will make clean up more difficult than necessary.