Thanks for all the kind words!
I was thinking that the third step could have a roll to decide if the spell succeeded or failed - because I think players will find it hard to just decide and tell another player that their spell failed. But I couldn't figure out what the probability of success should be without knowing the scale of the effect, etc. In the end, I punted and didn't mechanize that part - though I may update this if I figure out a nice way to do it.