This is how i marked mine: Unloaded, I marked the string on each side of the rail. Then when you load it, you can see if the marks are even on each side. If not, just pull back on the string alittle to even out. It definetly makes a difference.
Exactly how I set mine up. I've had half a dozen people shoot it, and it's pretty consistent from person to person, lefty or righty, it shoots pretty consistent as long as the string sits dead center like it's supposed to.