Wait. You a spontaneously casting a level 25 spell with only scores of 5 in the Technique and Form?
Looking at your numbers, it looks to me like you are not applying the "/2" to your casting roll for a fatiguing spontaneous spell. Seems to me it should be:
(21+5)/2=13, so 8 more Fatigue levels needed
I see nothing in the virtue's description that states you do not have to divide your total, as normal. Even Diedne Magic doesn't forego the division, so I see no reason why LLSM would.