I think its a good question, yeah its been discussed before, but honestly sometimes finding information on this forum in the search is pretty difficult.

There really isn't any specific rule of thumb for doing this. It really depends on how well the lace disappears.. If you want to wear your hair completely back, and the lace isn't really invisible on your skin 100 percent , you could cut it right as close to the hairline as you can. But generally people prefer to leave a little bit extra lace in the front. I like to leave a little bit because when the hairline frays you have some room to trim it off. For example.. As your trimming.. u can put the lace against a dark background.. like a black shirt or something so you can see where the knots are.. U can try leaving anywhere from one lace square to 1.5 to two empty lace squares in front of the knot. To give an approx of measurement.. get your ruler out.. find where the MM. millimeters are. I think a good amount would be 2 to 4 mm's of extra bare lace left at the front.. U don't have to measure it or be exact.. but just to give u a general idea of about the amount of lace you should be leaving.. and you can always tweak it. If you feel its too much bare lace u can cut it up to 1 mm etc of the hairline. Really the choice is yours.