Describing a numbering system that counts from x at the y position in an index

Question

From a theoretical perspective, I'm interested in learning how one would correctly describe a numbering system that starts counting from x at the y position in an index.

This came up when our team recently created a task list using a spreadsheet. Items are referred to by their line item; for example, the item on the third line is Item #3. However, the spreadsheet also has a header row, somewhat like this:

 1      TASK                    DESCRIPTION
 2      eat cheese              gouda is preferable here
 3      look around you         just... look around you
 4      rickroll                never gonna give you up
 5      spoon                   you spoony bard!

The counting and indexing system has these properties:

• indexing begins at 1
• the first item in the index begins at 2
• the nth item is therefore at the n-1 position

This is a simple enough system to understand at a practical level; but, how would one model it on an abstract level, and what are the correct terms to do so?