As I understand it, the weights of the properties only apply when you imbue, not unravel an item.
Unraveling intensities should be fairly straight forward, just divide the item's value by it's maximum attainable. Here are some examples:
any slayer = 100% (an all or nothing)
luck 80 = 80% (since it is 80 out of 100)
fire resist 12 = 80% (since it is 12 out of 15)
mana increase 7 = 88% (since it is 7 out of 8)
You can find all of the properties, their ranges, and what items they can be found on here:
Magic Effects Table.
Stayin Alive,
BG