RPM Cost for 1 second casting time?

[RogerBacon]

I'm wondering what the cost would be for a 1 second casting time instead of 5 seconds. There are various ways of calculating this and they seem to put the value in the 20-40 point range:

Calculation 1: +4 to casting skill (since you can reduce tiem by -1 per second). This is 16 points plus (other 8 skills as aternative 'powers' at 1/5 cost). Then slap on a -50% limitation, "only for energy accumulation". That comes out to 20.8 points

Calculation 2: As 4 levels of Compartmentalized Mind with an 80% limitation "Only for energy accumulation" it comes to 40 points.

Calculation 3: Working backwards to figure out the limitation on going from 1 second casting time to 5 seconds. This is about two and a half levels of "Takes Extra Time" limitation for -25% limitation. But -25% of what? Well, probably Ritual Adapt and (on average) 4 levles of Magery. A -25% limitation on that is 20 points.

I'm thinking of calling it 20 points and could even break it up as 10 points to make the first roll 1 second and 10 points to make subsequent rolls one second.

Other input/ideas?