Very nice bike. I see you already have the solo seat on it. I also bought a new 2013 B5 a few months ago, and currently have just over 2500 miles on it. I also keep my oil level just at the top of the sight glass, and have had no problems at all. All my life I have always filled engines to the top of the sight glass, or even with the "full" line on the dipstick. Never ever had a problem. I just can't deal with the idea of running an engine with a low oil level.
If you don't have any oil coming out of the crankcase breather, it is not over full.
You said it is coming from the clutch side when it is on the sidestand. Does it leak when parked on the centerstand? If not, the problem is caused by oil running over to the left side of the engine when the bike is leaned over, and likely leaking around the left engine sidecover. Don't overtighten the bolts. If it still leaks, it may need a new gasket. If you bought it new this should be a warranty issue.
Enjoy your bike. The Enfield is a very special one of a kind bike, and provides a very special one of a kind ownership/riding experience. It's the only bike out there that lets you experience the undiluted feel of riding a vintage British bike without actually having to deal with the expense and issues of the real thing.