All I can say is what Verhoeven said in his book. Thickness is also an issue to you may well get a 1/8" coupon to harden all the way through, actually I would sort of expect it, and a 1/4" coupon will not. Here again we have those annoying (or not) "auto-hammons" where the blade just doesn't seem to harden over around 1/8" thick. You can etch a blade like that and at the junction of the martensetic and pearletic steel the line will show up between the two steel phases. The ricasso will take on the etching characteristics of pearlite the same as the spine. Another thing that comes into the mix is grain size. Larger grain promotes hardening. If the grain is large enough then the 1/4" coupon will form martensite with a loss of strength due to the grain size.
As far as how one would get a measurement of hardness at 1/8" or thinner with fine grain, you would make a coupon of that thickness and take it through the hardening and tempering process that is being tested and measuring that. It's not just the W series of tool steels that act like that. Other shallow hardening steels like 1095 or 1075 will do it too. This is where the low alloy steels like the 1080 that is out on the market which is actually 80CrV2 or 1084 which is really 1084 plus a little chromium come in handy. They will harden past 1/4" thickness. 5160 steel would have the same relationship to 1060.
Doug