Great Work!!
This is Great!! Let me just say that this is exactly what I have been looking for in a Alt.Arcane type. I really do believe that this almost completely negates the need for a Sorcerer (though there are some things faster Mana generation can get you), but I always had such problems with the Sorcerer that that's fine with me!!
I took the numbers from your webpage and ran then through the Excel wringer to look to statistical stuff. I have come to a few conclusions.
1) Levels 1-2, 8 and 17+ cause a problem for Wizards, though for different reasons. Currently 1st, 2nd and 8th Level Wizards have double the chance of spell failure than any other, and level 17+ Wizards have a ridiculously low chance of spell failure.
a. Reduce the Mana cost for 1st level spells from 3 to 2. This allows a Wizard to cast a first level spell in two rounds (instead of three) which brings this level in line with other levels and this decreases the average chance of spell failure from 10% to 5%, bringing 1st level spells within the +/- 5% failure rates of every other level from 3rd through 16th. (I would also reduce cantrip to 1. Just for "Does this really make sense?" reasons I think cantrips shouldn't take 2 rounds to cast.)
b. Increase the Mana die at 8th Level from d4 to d6. This decreases the average chance of spell failure from 10% to 5% without changing the average casting time of 3 rounds.
c. Failure rates drop through the floor at 17th level to -30% and less
while at the same time casting time drops from 3 rounds to 2. This makes 9th level spells far easier to cast than any other level of spells, relatively speaking. If you want to bring 9th Level in line with all other levels reduce the Mana die 2d8 to 2d6 and reduce d20 to 2d8. This increase both the difficulty and casting time of 9th level spells.
2) Wizards are now completely linked to their Concentraion Skill Rank, reducing it from an option to a necessity (Just like the Ranger and Wilderness Lore, or Bard and Perform). They only get 2 SP/ level (less than the other two classes, and all Alt.Rangers have 6 SP/ Level to boot), and now one of them is always tied up (if they want to cast spells that is).
I would either
a. grant Wizards 1 SP in Concetration every level automatically, or
b. divorce the two by making a Hold Mana roll d20+Wizard Level v. DC of 2+Mana Held (which is statistically the same formula as the one you presented).
I think I like option
a. better because that allows a Wizard to take Feats to boost his Concentration score (at the cost of Metamagic and Item Creation feats of course

).
As a foot note, all of my Calculations were done assuming the casting of the highest level spell possible, not including bonus spells due to high intelligence. Casting higher level bonus spells of course increases the difficulty and casting times, while casting lower level spells than you are capable of lessens both.
Here's my revised progression chart.
Wiz Level: 1 3 6 7 8 11 13 15 17 19
Mana Die: 1 d2 d3 d4 d6 d8 d10 d12 2d6 2d8
Spell Lvl : 0 1 2 3 4 5 6 7 8 9 10 11 12 13
Spell Cost: 1 2 4 6 8 10 12 14 16 18 20 22 24 26
With these tables all average casting times (with the exception of cantrips and 1st level, which are 1 and 2 rounds, respectively) are 3 rounds, and all average spell failures are -/+ 5%.
Great sytem Eternalknight!! I hope you and others find all this interesting! Lemme know what you think. If you want the actual spread sheets (I did them in Excel XP) send me an email and I will send them to you.
Irda Ranger
irdaranger@yahoo.comirdaranger@yahoo.com