Darrell said:
Right now, I'm leaning toward linking it to the 'Magic Attack Bonus' in the Microlite20 rules.
I'd be inclined to go with a variation on this theme: Have a 1st level "Turn Undead" spell for Clerics where you roll Magic Attack bonus to turn, DC = critter's current Hit Points. That makes for cool tactical options - do we hit first to weaken it and turn later, or turn first? It's kinda cool to be able to fight down Huge Undead then explode him with a good Turn check
I'd also allow certain items of Religious Significance let anyone with the faith Turn Undead - in effect, they'll have their own internal HP that can be used to fuel the Turn Undead spell. The internal HP dictates how many times per day it can be used. The Magical Attack bonus of the wielder is used at all times.
Clerics would prize these very highly as they also give them free use of Turn Undead per day too, of course.
Example: Sun Shield of Pelor: able to cast Turn Undead, 6 HP (allowing use of Turn Undead 3/day), +2 AC, usable by Clerics of Pelor without Proficiency. Grants a +2 to Magic Attack bonus for Turn checks.
As to the Monsters, you've done it again, Darrell

I've put this up on
http://home.greywulf.net/m20 as a supplement. I'm inclined to keep the critters in the core rules too, at least for now, just so it's "complete" in two pages. I reserve the right to change my mind, of course!
We playtested having spells cost Hit Points = double spell level + 1 (0=1,1=3,2=5,7,9,etc) last night and it works very well, especially at higher levels of play. Our 1st level Mage with 15 Hit Points let loose 3 Magic Missiles against the Kobolds in the Burning Plague (our usual playtest 1st level 'venture) before pulling back to cower behind the Fighter, and still had 6 Hit Points to spare. Amazingly, he survived too
On the higher level test (15th level characters vs. a red dragon and his troll minions), the Mage with about 72 Hit Points used 3 8th level Incendiary Clouds (total HP cost 51) to decimate the trolls. Nasty. Of course, that only left him with 21 HP to face the young Red. He died pretty quickly afterwards - he should have used just one Incendiary Clouds and kept the rest in reserve for the Big Fight.
That's what I like about Microlite20 though - it gives choices like this back to the player, rather than limiting the tactics based on spell slots.
I'm going to put the Spell Cost to "double spell level + 1" next time I update it.
Let me know what you think.
Thanks!