Hi,
I see two issues in your formula.
First is the sheet name reference. You must use the same format as what you would normally find in a spreadsheet i.e. 'Calculations'! not Worksheets("Calculations") The formula bar is still treated as a string of text
Second, for this I suggest using VBA/Excels R1C1 referencing method
For counter = 6 To 23
Worksheets("Calculations").Cells(counter, 27).Value = Worksheets("Data").Range("G22")
Worksheets("Calculations").Cells(counter, 28).Value = Worksheets("Data").Range("E22")
Worksheets("Calculations").Cells(counter, 29).Formula = "='Calculations'!R" & counter _
& "C28 * 'Calculations'!R" & counter & "C27/1000"
Next counter
I also noticed that you had the counter start at 1 to 18 but in each line you always added 5 to it. You can just start the counter at 6 and go to 23 for the same results
Hope this helps,

DaveG