@ antywanty

Before attaching, if you position the piece exactly where you want it, lift the front part up and (using a makeup pencil) put 2 dots (approx 3inches apart) on the base, about 3-4 inches back and then corresponding ones on your scalp.
Once you've glued/ taped and go to place it, simply match up the dots.
It'll go back to exactly the right place.
Another tip, which many will already know, is to press the teeth of a comb all along the front hairline.
It will leave temporary indents on your scalp.
This will show you exactly where to put the glue.
Obviously the indents disappear, but by using the dots, you'll place it so that the front of the piece matches up to where you've glued.

But you're SO right about a slightly wrong attachment making a huge difference to how the hair falls, can be styled and your appearance.
Its usually recommended that the front of the hairline is about 3 finger widths above the eyebrows.
More if you're older.