I wrote it in a bad way - tree has to be 100, 200, 300.
This solution did not wor for me.
I think you are out of luck,
The incrementation is 1 digit at a time - not 100 - and there is not way to change this that I know of.
You will have to manually ID the subsets I think unless someone else has a solution.
If I understand correctly, you need your layout tree to be
If that's the case, then you need to set it up by using the first number as a subset, and that will carry down to another subset and then to the layouts.
[MASTER SUBSET]: ID assignment of items in subset as "1,2,3,..."
+[SUBSET 1]: Use prefix and ID of upper level (automatic 1), ID assignment of items as "01, 02, 03"
++[LAYOUT 01]: will have ID as 101
++[LAYOUT 02]: will have ID as 102
+[SUBSET 2]: (automatic 2)
++[LAYOUT 01]: ID will be 201
++[LAYOUT 02]: ID will be 202