By its wording you can see that this part of Diedne Magic should work the same way as a Magical Focus for fatiguing spontaneous magic, hence (ArM5 p. 45f and Errata for ArM5 p.46), if a requisite is the lowest applicable Art, that one is doubled for the casting total.
No, you always double after taking the minimum. It has been clarified in the errata, IIRC, due to confusion regarding the wording of major/minor focus. Diedne Magic works just the same:
I just inserted a column to the right of the casting total and put this formula
in it (got it from a helpfull person on the excell news forums) microsoft.public.excel.newusers --
Just do a search for Thrumbar on that newsgroup. Changed the formula
so it would work in the column I made to the right of the casting totals instead of the spont area which I deleted from my personal version.
D16 Form
E16 Tech
F16 Te Req
G16 Fo Req
H16 Spell Level
I16 Casting Total
K16 Affinity
I'm writing my own in C# to support my chat-based campaign It's basically a multipane relay chat client with special support for RPG communication and (Ars) character sheets.