A Few Questions About Spellcasting Power

[Sigma77]

(tips hat)

Long time RP'er and general DFRPG fan here. I had a question I've been trying to wrap my head around, I was hoping I could get some answers from the community.

• How do you handle drawing in power from the environment for an evocation? We've seen a couple of cases in the book where Harry's done it - Storm Front = Lightning Scene, Harry's Gravity Spell, Ley Lines?
• Do you let your players Tag aspects and spend Fate Points for access to more power than they could typically access without accruing stress? Like, say their Conviction is (+5), they got up to that limit, they could tag a Fate point for 2 more power without having to pay the extra stress?
• Do you let your player's split casting a spell across exchanges? There's an example for this in the Paranet Papers, via Propless Magic. My thought is if you could do it that way when you're hampered, it would make sense that you could potentially do it in combat. The obvious liability would be having to sit there in live combat spending your turn powering up, so to speak.

[MadAlchemist]

To the first question; I treat those as navel gazing maneuvers for Evocation. Maybe Lore to use the surroundings, Conviction to draw more from your personal well of power, Discipline to focus on the effect you are about to do, ect.     
To the second question; I'd say that you can boost your Conviction rating with tags to put more oomph into your effect. That effect you mentioned could be a 7 shift effect with only one stress. Maybe even a rote that the character needs prep time to get right. The control roll wouldn't be affected unless you have more fate points or tags waiting.
To the third question; I have yet to pick up the Paranet Papers, though it's on my list, so I don't know the specifics you are mentioning. That said, you can split casting over multiple exchanges when doing Thaumaturgy. It's dangerous, but you can and may need to with even moderately complex rituals. 
Not so for Evocation outside of the tags mentioned  earlier.

[Taran]

1. It's just scene aspects tagged for control or power - see below

2.  On occasion I'd allow using aspects to boost a spell without needing to use more mental stress but, no matter what, you still need to control the extra power.  I wouldn't allow a whole bunch of free spells...although, maybe, if you spent a FP on it, I might allow a spell at a certain Power without it costing stress.  Kind of how using a Ley Line might work.  (a la 'Places of Power' YS pg. 292; and 'temporary access' pg. 289)  So, I'd do it on a case by case situation.  But, yeah, you'd still need to try to control it.

Someone mentioned that using those aspects may even cause  you to pull in more power than you can control...which would be a cool compel.  Trying to channel a thunderstorm is dangerous...

3.  I'm not familiar with how that works...I've never tried it.