Referencing A Dynamic Range

1. I have a worksheet with subtotal and total row above all columns

I currently use SUBTOTAL(9, A6:A6000) and SUM(A6:A6000)

however the no of rows is dynamic based on imported data so I want the range end (i.e. A6000) to also be dynamic i.e. refer to the last cell with data in the column.

Something like SUBTOTAL(9, A6:LastCell in A)

2. I also use SUMPRODUCT to do calculations

e.g. SUMPRODUCT (($B$6:$B$6000 = "ABC") * ($A$6:$A$6000))

again how can I make the sumproduct dynamically use all rows to the last active row.

3. I have tried just setting the second part of the range reference to a very large number but am worried that this slows down the calculation. Does it?

e.g. SUMPRODUCT(($B$6:$B$100000 = "ABC") * ($A$6:$A$100000))