Mike. Try not to overfill it as this can cause pressure to build in the engine and risk blowing some oil seals.
How do you look in the sight glass (No with my eyes gags). How do you hold the bike level?? The thing is that if you've got a mate to stand and hold your bike it is probably the case that he hasn't found the balance point of the bike and could be holding it a varying angles giving false readings.
Kneel next to the bike by the footpeg facing towards the front of the bike. Now put your left arm over the seat. Get your mate to stand with hands on the bars but don't let him take any weight, he is there as a precaution. Now as you are kneeling looking at the sight glass, pull the bike towards you and off the stand, you will be able to feel when the bike is at its balance point.
This is where you should take the reading. I think you will find that you will not get a variation in the readings anymore. The oil level should be up to the top line next to the window.