Shimano 7 speed spacing = 5mm c/c between sprockets, 8 speed = 4.8mm, so over the 7 sprockets you're using you are only off a total of .2mm*6=1.2mm, which if you center this it is only 0.6mm max on each of the outside sprockets, less as you work in to the center. You may just need to loosen your cable a touch, and/or a slight adjustment to your limit screw.