The question in my mind is "Why assess, when you can measure and be more certain"?
I would reiterate the advice to use a Depth Micrometer, or the jaws of a digital, dial or vernier Calliper.
Using Slips is probably OTT, unless you really need to have lengths correct to within 0.0001"
Don't waste time chasing microns, needlessly. (Unless that is your hobby )
Industry will not work to microns, if 0.1 mm will suffice. The cost increases, possibly exponentially, as the tolerances decrease.
Howard