Promotional Goods Modifier to Apply for a specfic Qty

We have a business requirement.
For every 5 bottles of an Item brought by the customer he should get 1 bottle Free of the same item.
But the customer should  buy  a minimum of  20 bottles to get the Free  Qty.

I have set up a modifier  as Recurring  - Item Quantity - between 5 to 99999

Define details  Get Qty 1 (100 percent free) 

But this works right from 5 bottles.

Suggestions most welcome .

Thanks & Regds,

Athmaraman