Here's the thing: combat won't last in minutes and it is a nightmare for a DM to keep track of. An average combat should last 4-6 rounds. If it is lasting much longer then the party is using too many resources and the CR is too low for the party.
How long does the light last? It sounds like it a permanent light. So having to make a Will save with a d% every round minus the amount of squares is far too complicated with a DM having to keep track every round (when PCs can move in 3 dimensions).
Keeping the check to moving 20 feet closer makes it 6 checks (at 100 ft., at 80 ft., at 60 ft., etc.) as opposed to them moving and having to remember d%-squares. It is far easier to keep track of distance rather than time because of the additional up and down movement. Granted I did not define how the target moves closer but it could be maximum distance closer and that would depend on Swim speed.
Your way, if the PCs are 100 feet from the creature for one round they have a 99% chance of success (or 1% since I'm not sure it is worded properly).
Anything that is easier for the DM to use is better. Monsters are fun to kill but should also be fun to run. Having to make a check every round in addition to the other things the DM has to keep track of is not fun for the DM.
Here is the updated Lure:
Lure (Ex): The dire anglerfish has a bioluminescent lure dangling from a stalk coming out of its skull, which hangs in the square adjacent to the front of the dire anglerfish. It shines light in a 100 ft. cone. All living things in this cone must make a Will saving throw (DC 22) or be compelled to move their maximum movement toward the light. For every 20 feet closer to the anglerfish that a target is, another Will saving throw is made with the DC increasing by +2 until the target is in the same square as the lure. The save is Constitution-based.
Creatures in the square with the lure make a Reflex save (DC 22) every round to avoid the poison that drips from the lure. If they fail, they are poisoned and take 1 point of Strength damage. They also have to make a Reflex save (DC 22) to avoid touching the lure. The saves are Constitution based. Dire anglerfish are immune to their own poison and the poison of other dire anglerfish.
Touching the lure allows the dire anglerfish to make a bite attack as an Attack of Opportunity. The dire anglerfish can still make this Attack of Opportunity even when flat-footed.
Last of all, the dire anglerfish can make a Hide check to conceal itself behind its own lure.
Anyhow, however you decide to make the checks, just keep the poor DM in mind.
Next up will be the Dire Carp (but I'll put it in a new post since this one is getting crowded).
Debby