Help with instancemanager.count to reference last field of repeated, nested subform?

I'm attempting to use instanceManager.count to get livecycle to tell me the last instance of a particular field inside of the last instance of a particular recurring subform.

hierarchy:
Table 1 (Tbl1SF)
-subform (Tbl1SSF)  This subform is a repeatable row inside of Tbl1SF.  When a new instance of Tbl1SSF is created, a second instance of another entity,
Table 2 (Table2SF) is created.  In this form, another row is repeatable,
-subform (Table2SF2)  In this row, exists a field that I want to run some code once the user has entered data in the final instance of it.  This field is called engArch.

I've tried writing and debugging the code in a lot of different ways, but I can't seem to figure out the syntax for referencing this.  I also haven't been able to find any sample code that will do this.

What's the correct JavaScript syntax?

form1.Table2SF[ + form1.Table2SF.instanceManager.count + ].Table2SF2[ + form1.Table2SF2.instanceManager.count + ].engArch