Promo Modifier - how to limit by UOM e.g. YR

We have a promotion for a free 12month free license.

The first purchase of any license by a customer establishes a Renewal date for all subsequent purchases.

When the customer uses the promotion for the free 12 months license we would like it to take into consideration this pro-ration, i.e. if the renewal date is set for only 9 months there should be 3 months left on the promotion for a subsequent purchase.

We have set up a promotion modifier with a line limit, being an amount of 1 @ YR.

With the Modifier line referencing the inventory item and the related UOM.

When we use this modifier on a license with a service period of 9months the modifier limit is being fully consumed.