iTotalRecords retrieved by AJAX

I'm using infinite scrolling and server side processing. How do I retrieve the iTotalRecords after I've already displayed the first set of results. Meaning, if I have 500 results and I'm display the first 125, I want iTotalRecords to be updated after I've already retrieved the first 125. Why do I want to do this? Getting the SQL_CALC_ROWS_FOUND is much more expensive then getting first 125 records. So, I can do that with a second request and update iTotalRecords after displaying the records to my customers.

Thanks