Okay, so shall we go with the twelve uses a day, concentration duration, with the item maintaining concentration, and only usable by Theraphosa, then? When I ran the numbers before, I came up with a Spell Level of 20, and an Effect Level of 40.
Using a magnetite piece of jewelry brings us to a Lab Total of 76 without experimenting. Experimenting increases Fiona's and Sheelagh's help by 6, which takes us to 82 + die roll of 9 = 91. Done.
Now for the icky part. The Extraordinary Results chart. Die roll of 6 shows Side Effect. Die roll of 9 comes up with "The spell has a major side benefit. For example, a spell that transforms you into a wolf also lets you speak to all beasts while a wolf." This could be interesting.