Put b. hp in the damage formula deal and multiply it by a decimal, it will then do what you want it to do.
Ah, why not just tell him exactly what to put in the field?
For the formula for FF's Demi which cuts 1/4 of the enemy's HP use this in the formula field:
if b.hp >= 2; (b.hp * 0.25); else; 1; end;
What it tells the Maker is: If the target's HP value is higher than or equal to 2, multiply the target's current HP by 25% (that is, cut one quarter) - else, that is, if the target's HP is below 2, deal 1 HP damage.
The If check is included because Demi doesn't kill the enemy unless its HP are 1, and if HP are 1, it deals 1 damage.
Edit: Forgot to mention that I'm using Victor's Damage Limit script (
http://victorscripts.wordpress.com/rpg-maker-vx-ace/gameplay-scripts/damage-limit/ ). Since Demi caps the HP cut at 9,999, you should modify the formula like this unless you're also using the script:
if b.hp >= 2; [(b.hp * 0.25),9999].min; else; 1; end;
It changes the damage to "1/4 of the target's current HP or 9999, whichever is the lower value".