How to create a sub index column

Hi!

I have a table sort of transactions table. The rows, 11 at a time belongs to one transaction. I've created a index column with Modulo function so each group starts with 0 zero and goes up to 10 and starts over. 

I want to add a new column so the first group of 11 transactions will all have 1, the next 11 will all have 2 and so forth. What function do I need to accomplish this or what's the best way to approach this?

/Magnus