I'm not sure if I caught the gist, but please see if the workbook
attached in the Forum Thread could give some ideas.
If the goal is mark all initial Customer Nulls with a 1
and the very first nonNull customer, then you can use
OR ATTR([Customer])<>"Null" AND LOOKUP(ATTR([Customer]),-1)="Null" //currently not Null, but previous one was Null
This will require ID, Customer, and Time to be somewhere on the shelves
so that the "Compute Using" can be properly set to mark the first, do the lookup, and restart every ID.
Is your eventual sheet going to be a table or a graph?
If your goal was to mark them differently (first nonNull customer only if not preceded by Nulls, etc.)
please mark in the attached excel your desired output.