The formula works correctly (and indeed returns the correct value) if I enter it manually (or simply change RC2 to $B2 so it references the correct cell). I just don't know why it doesn't get converted from RC2 to $B2 when it's entered using FormulaArray.

Originally, I set this all up by manually entering the formula and confirming that everything worked OK. I then coded the macro to duplicate what I had done manually. But for some reason the "required" R1C1 format doesn't get converted to A1-style.