Are you using that strata setup I made a while back?
If so, you should be able to plug the output into a Distribution shader child and mask the subtraction calculation from there.
I'm doing something similar with something I'm currently working on, unfortunately I've got stretch marks where I don't want them, so I'm going to have to think of another way. Probably a lateral displacement but I need to work with normals now and dust off the old trigonometry head.
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