First, define exactly what you mean by overdue in terms of your data, that is, define the "business rule". That definition should be in words, not IF statement syntax, though you can say "if this is true then that and that and this and that", but don't concentrate on FMP/other syntax during that definition.
Once you define the rule relative to your data, the calculation usually falls right out.
I suppose that you just have one field that should calculate the overdue-days and you showed 2 separate calculations. The 2 can be forged into a case-calculation like this one:
not IsEmpty ( Obj 1A completed ) ; Update 1 due - Obj 1A completed ;
IsEmpty ( Obj 1A completed ) ; Update 1 due - Get ( CurrentDate ) )
Make sure that you select the "unstored" box in under the "storage" button in the calculation-definition
If ( not IsEmpty ( Obj 1A completed );
Update 1 due - Obj 1A completed ) ;
Update 1 due - Get ( CurrentDate ))
case could be be used in place of If. No need to do isempty twice.