Before you drain the fork oil, measure the oil level to use that as a reference. Not all factory levels are correct. If you leave the forks upside down overnight, you will be surprised how much junk will drain out. You should pump the forks a bit to get all the oil out of the internal chambers. The oil level will change a bit if you have progressive rate springs and turn them upside down because more, or less, coils will be under the oil.
If you take a glass of water and hold it at the same angle of the forks, you will see the distance from the oil level to the top of the glass will be different on opposite sides. Once you set the level where you like, then you can measure the same way from then on, such as down the back side of the tube when the forks are mounted on the bike.