The line is scribed to give a measuring point that is true with the wheel. You roll it a few tire rotations to let the suspension settle out after lifting.
You can't measure halfway up the tire because the body will be in the way. You measure as high as you can without deflecting the measuring tape with the chassis of the car and about the same height on both sides. You are measuring between the scribed lines. You adjust the tie rod sleeves to adjust the toe. Loosen the locking clamps then turn the sleeve to adjust toe. If you adjust half on each side you will keep the steering wheel centered. After adjusting MAKE SURE YOU RETIGHTEN THE LOCK CLAMPS.