Powers start at 2 points for Fast progression.
Extras add +0.5 to the power progression role-creation cost; flaws subtract -0.5. The exception is that 0.5 goes down to 0.25 (and it can't go any lower than that - there's a max of 4 net flaws).
Modifier categories:
Progression: Very Fast +1, Fast +0, Medium -1, Slow -2, Very Slow -3.
"Very Fast" increases one's effective adept level by one. I base this on the Companion's Supernatural Philosophies. If you expand a supernatural philosophy's Canonical and Barred powers until all powers in the core book are assigned, you end up with 39 powers that have a +1 to adept level, and no others. This is tantalizingly close to the number of powers granted by Broad access. So if supernatural philosophies are balanced (and I'm inclined to think they are), then it would seem that "+1 to adept level" and "Broad access" cancel each other out... and thus Very Fast is a single extra.
Access: Unlimited +0, Broad -1, Narrow -2, Singular -3.
If there are multiple power-using roles in a setting, and one has access to powers that are normally restricted, that might be worth a +1 if the restricted powers are large in number or especially juicy. (Note that "Unlimited" doesn't really mean "unlimited" - it just means that a role gets more powers than is appropriate for Broad.)
If a role can't buy powers, but only builds on progression-level from other roles, that's a -2. (You'll want the Access level set the same as or higher than the other role(s) - otherwise only portions of your powers will progress.)
Consequence Saves: I have used this in previous threads to designate the Fatigue save, along with any other alternatives to it that power-using roles might use. They all involve a save vs. some negative consequence happening when powers are used. Conviction can be used to automatically avoid the consequence unless otherwise specified, just as Fatigue can be cancelled out.
The save used is Normal for the role: -1
No save, consequence automatically happens unless Conviction spent: -2
Consequence can't be eliminated or avoided with Conviction (other than by rerolling the save): -1 (-3 if combined with the above)
Consequence significantly worse than Fatigue: -1 (or -2 in extreme cases)
Consequence penalties apply over one day, not one hour: -1
Consequence penalties build up indefinitely, unless eliminated with Core Ability or other nontrivial means: -2
Consequence penalties build up until consequence happens, then reset: +0 (Note that using Conviction to evade the consequence means it didn't happen! The Narrator may choose to let the penalties decrease very slowly if all power-use is avoided - at most one point every couple days.)
Use Restrictions:
Spellcaster - speech and gestures needed, armor check penalty applies to power checks and consequence saves: -1
Focus - speech, gestures, and focus needed, no armor check penalty: -1
Situation dependent - activation of Dedicated or Higher Purpose, for example: -1
Can use ritual magic: +1 (If every power-using role in the setting can use ritual magic by default, don't apply this; instead apply a -1 to those unusual roles that *can't*.) If built into the role, one does not need the Ritualist feat.
Can ONLY use ritual magic: -1 (includes the cost of the above - so -2 if that extra doesn't apply)
Flexible - Can trade in a Fast power for two Slow powers, a Medium power for a Slow and a Very Slow power, or a Slow power for two Very Slow powers (Slow only): +1 It also works in reverse; that is, if you buy the same power as Slow twice, it becomes Fast. You can't make a power 'faster' than your base progression speed.
Can use The Talent to gain powers outside one's Access: +1
Powers vary in key ability: -1 if key abilities are fairly evenly distributed between powers.
Many, many more are possible; it's impossible to enumerate them all. Usually it's best, I think, to eyeball any odd factors and assign a number to them in toto, rather than trying to tally them up separately. You can see two examples of more complex eyeballed flaws here.