Effects which add a stress die in Enchanted items

I have a Magus who, worried about his mundane retainers being affected by rego mentem effects, has decided to make an enchanted item with the Unraveling the Fabric of Mentem spell.

The spell description says it will dispel Mentem magic of level+10+stress die (no botch) - does this stress die need to be rolled only when the item is created and the enchantment then has a fixed level it can dispel, or is it rolled every time the item is used?

I would say it needs to be rolled every time the effect is activated. The die roll is a slight randomizing mechanism for the power.
If the die is rolled and the number fixed some enchantments would be better than others. After all, if casting the same spell you roll every time, since there is a degree of chance there as well.

I personally use: "in an enchanted item, die is 5" for those "effect needs roll" guidelines.

That fits the theme with fixes Penetration value as well as being simple and quick.