Hi martinlofqvist,
Nice that it works for you.
Yes of course I will answer your questions. :o)
“Subtract by 1 to compensate for the array starting at 0?”
Yes exactly. As the Count is a total number like three , but the index number would be 0, 1, 2 for the three.
To the best of my knowledge, in the early days, memory space was expensive and the first bit 0/1 would be wasted to 50% if the zero wouldn’t have been used. Since then, often but not always, things start with zero.
“Why do wee need this Node? The output from it is 84 - same as from the subtract?”
The output from it is zero to the maximum value. It counts one time through the complete series of numbers, each time a new frame is given during rendering. While editing a scene typically while the scene is refreshed. In this set up it goes so fast that you see only the maximum value.
We need this Node, to create a individual scenario for each ID of Each clone to be processed. If a number is given out, all the connected expressions will be calculated or executed with this number, If it is done, the “Iteration” moves a step forward to the next.
In this way, we create an situation that “digests” for each clone and its selection entry in the tag “Index/Selection”, step by step.
“The Formula compares which input is greater and the result get computed? Q: Could not this been a Compare node?”
Yes, that is exactly what one could use as well. The compare node is flexible enough to do so. I use the Formula node based on its flexibility and that I can expand it later on without rebuilding parts of the scene. Perhaps one more input and another “If - Then” condition… Again, yes, compare would have been possible here.
“Feeding the result from the Formula and the value from the iteration. I don’t understand how this works…”
Think of a simple excel sheet where you have values in organized based on index numbers, 0- 84 for example.
the values in this tag is simple: Selection Tag feeds the “excel-sheet” into the Node.
This sheet is then addressed row by row. Which would be the index number, counting from “Iteration Start to Iteration Finish”.
While it is at a specific number the formula decides which number or row should be come a “Selected” Not select equals zero and selected equals 1.
Since you wanted to have only one clone scaled up, the only time the entry is “one” or in Boole terms “True” is when the “Controller” is equal to the “Iteration”.
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I hope this gives you a starting point to explore it again and built your own set ups ASAP.
Let me know if there is something not clear, I will try in an other way then, until you feel comfortable.
Often when I was asked how to learn XPresso, as you did above, I answered that the first thing is certainly to know the difference of Integer, real, etc values, then Vector and Matrix, Local and Global as well String and Boole.
There is certainly more, but this is the core information that flows through the wires of Xpresso.
Also, have a look on the left of the XPresso Editor, the X-manager. THese entries should flow in sequence, will say from top to bottom, or how the information flows. In other words, is the inforamtion available when it is needed? This is often figured out by XPresso, but to support this flow is certainly helpful. Just drag the entries. It helps me to think it through again and find maybe mistakes I did earlier. 
ENJOY
