I mount my quilts farther to the right. When using robotics, the machine goes from left to right. I measure to the rail, and after the first pass I always have to stretch out the right side to get it to be where it needs to be.
If I use the channel locks to work my way down, each pass will end up a smidge farther in than the one before. I assume it's just from the quilt being drawn up because of the batting. I will use the channel locks to check the center and along the front edge, and I can use it for the left edge after I've gotten the right edge and center looking good. But I still prefer to measure from the frame end instead.
Example, I'll load a quilt and measure and the top will be 14" to the side frame rail. I'll quilt and roll, and then when I measure the back edge, it will be 13 3/4" and I have to tug it a bit and get it out to 14". Then I'll put the clamps on the back, and measure and pin the quilt top to the back and the middle and front edge. I never baste the top or sides - gave me too much trouble when putting on the binding so I quit doing it. I pin instead, and remove the pins as I go.