There is technically no way to make this measurement check on a rivet joint that has a dimple countersink because the measurement is done adjacent to the rivet shank. You can not measure with a feeler gauge at the bottom of the dimple.
With a dimple in a dimple there is likely always going to be a small amount of gap between the layers right adjacent to the dimples because of the way they nest together. This is not of concern because a lot of the shear load is being carried but the dimple acting within the other dimple.
The check you are referencing is for two flat sheets when all of the shear load is reacting on the rivet shank.
Short story version.... if your dimples are properly formed (that is an entirely different subject in and of itself) you do not need to be doing any gap measurements.
A different but somewhat still similar (only for different reasons) subject that may be of interest is in the sticky thread linked from the top of the forms page.
https://vansairforce.net/community/showthread.php?t=159343
The thread is related to a dimple in a countersink but it is similar to what is at play with a dimple in a dimple as well, except it is because of dimensional differences between the bottom of one dimple and the top of the other dimple. In particular look at post # 24.
Some builders will likely say, "that is why they use sub structure dimple dies".
So that the bottom dimple is large to properly accept the dimple in the skin.
I do not agree with this practice because to make a bigger dimple with the same #40 hole, the finished hole diameter has to get larger.
The ratio of the diameter of the hole to the diameter of the finished rivet shop head has an influence on the overall strength of the rivet joint.
When you dimple a #40 hole with standard dimple dies the hole diameter enlarges to be right at the Mil Spec limit for a 3/32 rivet. If you use substructure dimple dies, the hole diameter is being enlarged beyond the maximum hole diameter specified in the Mil Spec.
Link to Mil Spec document on Van's web site
https://www.vansaircraft.com/faq/mi...pec-rivets-buck-type-preparation-and-install/
The Mil Spec, is the reference used for hole sizing, recommended rivet call outs, Construction manual instructions, etc., by engineering on all the RV kits.