Whoops, that's my fault. Must have missed that somewhere along the line.
If you're not in any particular hurry to cast the spell, then you can just do it without rolling, so if you went back to Ziggy's place to do this then we go "yes that sure was incredible spellcasting what a budget this show must have!"
If you're doing it in the station, then presumably you're more concerned with time. At that point, we keep track of the number of exchanges it takes to cast. Each exchange, you determine how many shifts of power to put into the spell (taking stress only if you go over Conviction) and roll Discipline to control it. Once you have enough shifts for the spell, it goes off. If you muck up a control roll, you either eat all the shifts you've put into the spell so far as backlash (mental stress for you) or fallout (environmental problems). If any of it escapes as fallout, the spell breaks down, but if you choose backlash instead it can keep going. So you can theoretically muck up a control roll on the last stage of casting, take it all to the face as backlash, and finish the ritual with no problems.
Casting it at a rate of three shifts per exchange (which is pretty feasible with your Discipline of Great) gives you a six-exchange completion time with not too much risk, which I think I pegged earlier at about three minutes. An alternate plan for doing it faster was for Phil to pump as much power as veekie was willing to risk into the spell up front and eat the backlash, then have you finish it off more safely by channeling one or two shifts per exchange for the rest of the ritual.
