You should be using grey, solid colored (and not gradients) segments to denote the areas within your greyscale image. The gradient property works by looking at the greyscale tone of a secondary image whereby lighter areas will be shattered first.
If you do the math 256/8 (256 different greyscale tones) divided by 8 pieces dictates that each piece should have RGB values with a difference of 30, starting from 255,255,255 for pure white.
There should be rounding off errors but that's where you tweak the Threshold values.
You should end up with 8 pieces with solid greyscale tones. Precomp this greyscale image and nest it into the shatter comp.
You then KF Shatter Threshold to total when a piece shatters, or when the next piece shatters. You will require a bit more math - 100%/8 (Shatter Threshold divided by number of pieces)= approx 13%. WHere increments of 13% (approx) will shatter the next darker shade of grey.
Intensive AE & Mocha Training in Singapore and Malaysia
Adobe ACE/ACI (version 7) & Imagineer Systems Inc Approved Mocha Trainer