So, I have come to the conclusion that spells function like this: Every spell has a specific damage range for a specific intelligence value. For Example, the spell flare, according to the T4C Bible does ~130 damage with 200 Int. With 300 Int (note: because there are noticeable differences between damage results, I will use the ~ (approximate) value) it does ~150. With 600, ~200. With 1000, ~250. So, we come to the conclusion that every spell, according to your intelligence does an X amount of damage.
Now, after this damage is found, power comes into play. It acts as a percentage increase in this damage. So for example, flare with 300 int does ~150 damage. If the char has 200 power, it will do 150 * 200% = 300 damage. (Remember that having 100 power means having +100 from the original 100, thus 200).
Resistances act in almost the same way. After this total sum of damage is calculated, we divide it with the percentage value of resistance. Example: A mage with 300 int and +84 power hit a warrior that has +50 fire resistance with flare. The total is: (150 * 1.84) / 1.5 = 184. So, in my opinion, the mathematical equation to find spell damage is:
(dmg of the spell in the specific int) * power (percentage)
-------------------------------------------------------------------- = dmg
resistance (percentage)
A few more notes. Intelligence acts as a "container" for power as the more you increase intelligence, the more you gain from having more power. For example, let's say we have 2 mages with 80 power each. The first has 300 Int while the second has 1000.
Flare does ~150 damage with 300 int and ~250 with 1000. The first mage's spell will do 150 * 1.8 = 270 damage while the second's spell will do 250 * 1.8 = 450 (these are results in 0 resistance targets of course). So I believe that cloak of the ages and many items that add power in general are a bit useless for characters with relatively low Intelligence/Spell Damage and REALLY useful for mages with high Intelligence/Spell Damage.
A classic example for that are Tank mages with 300 Endurance and 300 Base Intelligence who have so much power-giving items on them which do almost nothing beyond one point. For example, they could carry a shield of the ages instead of a power focus (the focus would give them, including resistances et such, ~+20/30 damage while the shield would give them very good AC)
Something very very important is to try and find the equation that would give us how much damage spells do according to the mage's Intelligence. This is a tremendous amount of work that few GMs have the time for, unfortunately :-(. In any case, my speculations are that for every 100 Intelligence above the spell's base requirement, the base spell damage goes up by 8-12% (not sure of course). From the tables, I believe we can get some results. Tsunami, for example, at 1000 Intelligence always does ~310 hp of damage, Inferno ~270, Flare ~240, Soulstealer 140 +140 and so on.
One more conclusion that I have come to is that, PROBABLY, inferno is flare with +10 - 15 power and tsunami is inferno with +10 - 15 power. Just speculations of course.
Of course, the difference between mental & physical spells must be made clear. Mental Spells (Shatter, freeze, mana burst, vortex of air, flare, dark spells, inferno, tsunami etc) are not AT ALL affected by AC. On the other side, physical spells (boulders, meteor) are, besides resistance, affected in a great degree by your opponent's AC.
The speed of spells maxes out at 1 hit/sec, except mana burst that maxes out at 2 hits/sec. when it reaches that speed, it goes no further even if the character reaches higher levels. Boulders for example (if I am not mistaken) maxes out at level 120-130 while the speed of meteor at that lvl is 1hit / 2 sec (almost - half that of boulders). Meteor maxes out at level 167-170. Tsunami at 158-163, burst near 110-120. Now, as a level 173 character I have all spells at the same speed, 1 hit/sec. If they were to be faster, then the spells that max out at lvl 120 would be now at 2 hits/sec but nothing like that occurred :-)
These are my notes, conclusions and sometimes, speculations. Whatever corrections or notes come to mind, please mention them so we can finally come to some solid arguments about this subject.
I really hope I helped
Hawk
P.S. I'd like to thank all of the people that helped me in my research (Clio, Tempest, Ekati, Harvester, Johny, Feingel, Ebonflame. Sorry if I forgot anyone :P)
