To be honest you should check the paper. I have never encounter paper to be the exact true cut as well as exact measurements for easels, some can be off as 1/8. If you follow the 5th blade and place the paper equally between the numbers of say, 8 or less than 8 at both ends you will be a lot more accurate on you borders.
Another way to solve your border problems is get a metric rule. Measure from the edge of the paper to where you want the border to be, 3 times on each side of the paper. Draw a line and connect the dots. This will give you your desire borders. Place the paper on the easel, center the paper on the 5 blade; take all you blades and place them on the edge of your line. This will give you the best accuracy for borders every time. Make several of these guides vertical and horizontal for every paper and and border size you may want.