I had to remove my forks to drain them, no drain screw on mine. I did have them vertical when I added and measured the oil level. I understand what you're saying, if the forks are on the bike, the are angled, therefore the oil level will be angled and read differently if you measure from the forward rim or the rearward rim of the fork. If your forks are already on the bike, I would measure each fork from the same side, front or back, just do them both the same. I think it's more important to get them as close to the same height than getting an exact mm distance from the top of the fork.