I am doing the BIA financial RTO and non-financial RTO how I can determine the Backlog RTO for some business units?

Just calculate the difference actual RTO - lapsed time.

I am curious to find out why do you want to calculate the backlog RTO, in case if RTOs already been defined and has there been backlogs that means those units should already be critical or reaching. I just want to get more clarity to answer appropriately