Nintex Forms - Calculated value lookup last/latest entry for multiple items

Hi all,

Google has not been my friend on this.

I have a single list that I'm using Nintex forms to lookup previous entries for.

I have a single field that the user enters the search criteria (a username) and then several calculated value fields use a lookup formula to populate previously entered information.

If I'm targeting a single user in the list (via his username) that has multiple items added, is there a way to lookup the latest item?

My formula is 

If(isNullOrEmpty(RACF),"",lookup("UserDB","Phone_User", RACF,"PC_User"))

This brings works however brings up only the first entry for that user instead of the desired last item for that user.

In my head, my thought is to call the user and sort the last ID/date modifed entry.  I can't seem to get that to work....if that's even possible

I tried a method that calls on Item Child Count, which seems to be 0 for all entries