Wallpaper and Wallcoverings - Wallborder- hanging straight

I have painted a room below chair rail height using a level line. I then stenciled a prepasted wall border to go above it. My problem is this - I created a level line 6 inches above the paint line - however when I measured it the line goes down - a lot ( around 2 inches from one side of the room to the other) Am I doing something wrong? Should I use this level line and not worry about the actual distance from the floor or should I make it equal distance from the floor and not worry about level? This is my first wallpapering type experience and I don't want it to be too messed up!
I appreciate any help!

I feel your pain....LOL....My old home drops 4 inches from the back to the front.When the drop is less than an inch I run it with the floor or base.Even though not level,it looks level in most cases.A level border may look out of level if the base drops or rises much.I prefer it to look level than be level.Now, 2 inches is beyond what you may be able to control.I would start it level but am sure it will look off.If it looks bad,about all you can do is split the difference,run it perhaps one inch out and take a look.I always try to keep in mind that furniture my have to be creatively placed to help.