I need to round down the following formula to one of a group of specific
numbers: (30, 27, 24, 21, 18, 15, 12, or 9)

ActiveCell.FormulaR1C1 = "=(r6c3)-1 3/4"

so if the number in (r6c3) is 24 then i need it rounded to 21
.... if the number is 22 then i need it rounded to 18.

How can i make that formula