The spell power is 1, the HP% parameter (Column E in my table) is 100.  It does 100% of your current HP 
in damage to the target.  What does it mean for spells that have spell power greater than 1?  There are an 
awful lot of spells that are hp-based that I didn't know about, and it doesn't seem like the algorithm is 
as straightforward for these.

There's a Spell Power stat and an HP Factor Stat (whatever you think of calling it).

When both are multiplied, they equal 100%, although I think that's not entirely required.

Hold on.. revising algo first...

For the SNES+GBA version, it is simply this:

Damage = maxHP/ Spell Power * Elemental Multiplier

Elemental Multipliers:
Weak vs Element = 2
Strong vs Elemental = 1/2

If you absorb this element and are using the SNES version, then apply against my magic damage algo.
If you absorb this element and are using the GBA version:
If single targeted, use my magic damage algo, but substitute the second value in the list as the spell power.
If multitargeted, simply invert the damage by multiplying by -1.

Maser: 1/10
Beam: 1/10
Fire: 1/5
Blaze: 1/4
Blitz: 1/2
Thunder: 1/4
D.Breath: 1/4
Big Wave: 1/4

HP Power (HP attack based on caster) = Caster's Current HP
HP Power (HP attack based on target) = Target's Max HP

Base HP Spell Power = Whatever is normally listed in the spell power slot
Base HP Absorb Spell Power = Whatever is normally listed next to the spell power data (**default: 100/Base HP Spell Power)
****Base HP Spell Hit Rate = Whatever is normally listed in the hit rate slot

**Note: Not sure if that always have to be defined like that, but it is there anyways...
****Note: Make sure to subtract 128 if it uses the boss bit

HP Spell Power Elemental Modifier (HP attack based on target):
Target is Weak to Element, HP Spell Power = 2
Target is Resistant to Element, HP Spell Power = 10
**Target is Immune to Element, HP Spell Power = 10
**Target Absorbs Element, HP Spell Power = Base HP Spell Power * -1
Unapplicable, HP Spell Power = Base HP Spell Power

HP Spell Power Elemental Modifier (HP attack based on caster):
Target is Weak to Element, HP Spell Power = Base HP Spell Power * 2
Target is Resistant to Element, HP Spell Power = Base HP Spell Power / 2
**Target is Immune to Element, HP Spell Power = Caster's Current HP
**Target Absorbs Element, HP Spell Power = Base HP Spell Power * -1
Unapplicable, HP Spell Power = Base HP Spell Power

**Note: In the SNES version, Immunity has precedence over Absorb. In the GBA version, Absorb has precedence over Immunity.

HP Based Damage Bonus:
HP attack based on target, HP Damage Bonus = 0
HP attack based on caster, HP Damage Bonus = (Caster's Current HP/ HP Spell Power * rand (100, 150) / 100) % 256

Basic Formula - Includes Weakness, Resistance, and Immunity (SNES and GBA versions):

//skip loop if using the GBA version...

while (Spell Multiplier > 0)
{
 if(Target's Magic Defense Multiplier == 0)
 {
  Success = true
  break;
 }

 while(Target's Magic Defense Multiplier > 0)
 {
  Target's Magic Defense Multiplier--;
  if (rand(0, 99) > HP Spell Hit Rate) || (rand(0, 99) < Target's Magic Evade))
  {
    break;
  }
 }
 Spell Multiplier--;
}

if (Success) //only applies to SNES version
{
 Damage = Base HP Power / HP Spell Power + HP Damage Bonus
}


Absorb Formula (SNES version):
Use Base HP Absorb Spell Power and plug that into the general magic algo as the Base Spell Power

Absorb Formula (GBA version):

If the spell is single targeted...
HP Absorb Spell Power = Base HP Absorb Spell Power * 3/2
Target's Magic Defense = 0
Target's Magic Evade = 0
Plug in HP Absorb Spell Power into the general magic algo as the Base Spell Power

If the spell is multitargeted...
Damage = Base HP Power / HP Spell Power + HP Damage Bonus