Question re: Ritual Magic

[Faolyn]

So I'm working on a GURPS Ravenloft game, and I'm trying to interpret the magic of Halan Witches. I'm using Ritual Magic and Power Investiture, but I want each college of magic (which I'm rearranging, rather than taking the standard GURPS colleges) to use a focus.

The focus would be an item with the appropriate Gadget limitations, but does the limitation go off the cost of Power Investiture, or the cost of the various Ritual Magic college skills, or what? The focus is require to cast spells, but not just to being a Witch.

Also, I'm using other limitations such as Time Required, would these go off Power Investiture as well?

Thanks.