Partial Potion Drinking Idea?

[The Glyphstone]

I just came up with a minor and potentially interesting idea off the top of my head. Right now, potions are an all-or-nothing deal - you have to drink every last drop of the bottle, at which point the spell is complete. It's fine for game balance, since if you didn't have to drink the entire thing, munchkins all over the place would be taking teeny tiny sips of their potions and breaking WBL into pieces - but there are rules for partially used wands, so why not semi-consumed potions?

Partially Drunk Potions

Sometimes, an adventurer has need of a magical potion, but only for a short duration - the spell in question might take effect much longer than would be necessary or desired. Consuming less than the total potion's contents allows for the liquid to be rationed out, but with a marked decrease in the magic's effectiveness. A potion is always consumed as a standard action, but the amount of potion consumed changes the effect's duration:

-If the entire bottle is drunk, the spell functions as normal with its intended duration.
-Drinking 3/4 of a bottle allows the spell to function, but at one "duration level" shorter than normal. If the spell has an instantaneous duration, its effect is 3/4 of normal.
-Drinking 1/2 of a bottle allows the spell to function at two duration levels shorter than normal. If the spell has an instantaneous duration, its effect is 1/2 of normal.
-Drinking 1/4 of a bottle allows the spell to function at three duration levels shorter than normal. If the spell has an instantaneous duration, its effect is 1/4 of normal.

Duration levels are defined as followed: Hours/Level --->10 Minutes/Level --->1 Minute/Level --->1 Round/Level --->1 Round. A potion's effect can never have a duration shorter than 1 round unless it is already instantaneous.

Example 1: Bob the Halfling Rogue is attempting to disarm a Fireball trap. He's fairly confident in his skills, but doesn't want to take the chance of becoming Roasted Rogue if he messes up. There's a Potion of Resist Energy in his pack, and he'd normally just drink it and get to work - but the party has seen small piles of red scales in the hallways, so they know there's a dragon around somewhere that Bob would prefer to have a backup defense against. He take the potion, but only drinks half of it, saving the rest for later. This reduces the duration of Resist Energy by 2 steps, from 10min/level to 1 round/level, easily long enough for him to disarm the trap or get Fireballed.

Example 2: Unfortunately, Bob wasn't as lucky as he hoped, and got a bit singed by the fireball. His Fire Resistance helped block most of it, but he's still down a few HP, and rather than bother the cleric for a healing spell, he decides to drink some of his Cure Moderate Wounds potion. Being almost full to begin with, he takes only a small swig of the drink, consuming 1/4 of the drink and leaving the rest for later. This heals 2d8+3 *.25, an average of 3 HP instead of 12.

Example 3: Bob again, but this time he's not doing anything so heroic as raiding a dungeon and killing a dragon. No, he's trying to reach the top shelf of the pantry, and lacks a Ring of Jumping +20 to do it with. What he does have is a Potion of Enlarge Person, but what if he gets hungry later? Drinking half of this potion will give him 1 round of Medium size, enough to grab his snack, and leaves enough liquid for one more swig later. However, he could not drink 1/4 of the potion to save the majority for a fight the next day, because three duration level drops would reduce the 1 min/level Enlarge Person to below the 1 round threshold.

It'd add more bookkeeping, but some people don't mind that sort of thing. The question is, how can it be broken or abused? Dischargable spells like Protection From Arrows come to mind, but there's not a whole lot of them...