Read entire chapter from the Faux Brick Book:
Are you half a bubble off?
Before starting to measure and draw in your faux painting brick painted wall, be sure you have something to keep your faux bricks level. Really, don't try to draw in faux bricks without finding a level and plumb line for your walls. It's like trying to tile your floor without finding square first. No matter how you try, if your first lines aren't square (level and plumb) you're not going to end up with a professional realistic faux brick wall.
The first line of faux bricks starts at the ceiling, or if your faux brick wall isn't going as high as your ceiling then you will be starting at the top of your faux brick painting area. The awful truth about houses is that the ceiling and floor is probably not level. Lay your level down on the floor, and see where the bubble lands. If that bubble is smack dab in between those two lines, you got level on your floor.
If you have a laser level, use that up near the ceiling to see how - Off - the ceiling is.
If it's only a tiny bit on a pretty long wall, say over 10ft, then your wall is going to be okay without using much of the fudge factor. Use your big 3ft level or snap a line with a chalk snap-line and this will give you a good visual. No problems?