How to Add Values in Average Formula

[JK1234]

I have a formula that I would like to add the values of another cell.

I want to use the F60 and F87 values in the average formula (N3) so that it will replace the F60 and F87 already in it, beacuse The values in R5 and R6 may change depending on which dates I want to use as my limits or is there an easier way to do all of this?? And these are the values I want to do the averaging (I will also want the same for the dev (CELL N4)

[JK1234]

I am resending again....