Adding "Lastrow" function to these formulas:

Hello,
I've been trying for a few hours now to add Lastrow Functionality to a formula I'm using as it's cycling through over 100,000 empty cells for each formula and making my Macro very very slow. My spreadsheet will have a highly variable number of rows.

Here's the formula I'm using, along with the Lastrow code that's served me will in the past:

lastrow = Cells(Cells.Rows.Count, "G").End(xlUp).Row

Range("AB3").Select
ActiveCell.FormulaR1C1 = "=ColorFunction(R3C27,C7:C7,TRUE)"

For anyone who might what to know what this is for (bigger picture), I'm using this code from Ozgrid to count and sum based on font color:
http://www.ozgrid.com/VBA/sum-count-cells-by-color.htm

Thanks so much for everyone's help in advance. I've been working on this for too long, I think my brain is going numb (yet it doesn't seem that complicated...?)