How about

CASE
  WHEN `Fiscal Year` = `Year Selector`
    AND `Month Selector` = `Month Name`
  THEN 
    SUM(`Amount`) 
    / 
    SUM(CASE WHEN `Class 3` = 'Sales' THEN `Amount` ELSE 0 END)
END

I didn't realize this was on a pivot table. Yes, the calculations must be full aggregated with the case logic contained in the aggregate. I don't have a sample dataset to test with. How about this?

MTD:

SUM(
  CASE
    WHEN `Fiscal Year` = `Year Selector`
      AND `Month Selector` = `Month Name`
      AND `Class 3` = 'Sales'
    THEN `Amount`
    ELSE 0
  END
)

MTD %:

CASE
  WHEN SUM(
    CASE
      WHEN `Fiscal Year` = `Year Selector`
        AND `Month Selector` = `Month Name`
        AND `Class 3` = 'Sales'
      THEN `Amount`
      ELSE 0
    END
  ) = 0 THEN NULL
  ELSE 
    SUM(
      CASE
        WHEN `Fiscal Year` = `Year Selector`
          AND `Month Selector` = `Month Name`
        THEN `Amount`
        ELSE 0
      END
    ) 
    / 
    SUM(
      CASE
        WHEN `Fiscal Year` = `Year Selector`
          AND `Month Selector` = `Month Name`
          AND `Class 3` = 'Sales'
        THEN `Amount`
        ELSE 0
      END
    )
END