I really hate the way Grimoire works. Backfires happen far more than just 15 % of cases, more like 50. Lately I had three backfires in a row on Conjuring Baked Goods. 0,3375 % chance to that, so it is said :(
Also: I played Spontaneaous Edifice, It backfired on me, costing a Chancemaker. So I quickly refreshed and tried again. Backfires again, and again costing me a Chancemaker. I tried this 4 times in a row, it would backfire every single time AND cost me a Chancemaker every single time. Chance for that: ((15 in 100) * (1 in 15)^4 = 1 in 100,000,000. So I finally tried to cast another spell before trying Spont Edif again (knowing by now what is to be expected). I cast Force the Hand of Fate and guess what? It too backfired. I don't believe this. There's no random in backfires. Backfires are hard coded, and more frequent than 15 % of cases.