Some high end easels (like my 20x24 Saunders) have adjustable left-hand paper stop for each paper size. This lets you center the paper to the scale. Otherwise you may have to make adjustments to the readings of the scale on each side.
Originally Posted by Cybertrash
Things to try
1) Get a new scale sticker; remove the old one and put the new one in the correct location
2) See if alignment of the paper with the RIGHT side of the slot works better (this solved the problem with my 16x20 Beseler easel)
3) Use tape, plastic or wood to make a small shim to adjust the right or left hand side of the slot to center the paper to the scale
4) When all else fails, figure out how far off the scale is and offset the blades the appropriate amount from the indicated scale readings each time.