While the standard first layer test evaluates the surface, this test evaluates the second layer from the backside.
The closer the nozzle is to the bed, the more the second layer will assimilate with the first layer, and we aim to target for a distance just short of this not assimilating with the first layer. I think it would be easier to understand the specific state to aim for, and easier to adjust.
In images A and B below, the second layer has assimilated with the first layer, and C has begun to assimilate. I think a little before C is best. (In a typical Fast Slayer calibration test, everything from B-E is in the proper range).
The goal of this test is to get the same results in all 9 spots.
Print instruction
Notes:
My Other Test Model:
Practical Overhang Test - https://www.printables.com/model/327913-practical-overhang-test
The author marked this model as their own original creation.