Okay so I never did this fight before, but I did look up a Youtube video...
The best way to do this, is to formulate a strategy. Come up with a formula and apply it, rather than looking at a chart.
You are working with single numbers and you are told to either divide by 3, 4, or 5 or make your number a Prime Number and there's four numbers (1 through 4).
Obviously, the primes are 1, 5, 7 and 11.
Divide by 3:
3, 6, 9, 12
Divide by 4:
4, 8, 12
Divide by 5:
5, 10
It's...... really that simple.
When he applies your debuff, look down at your number. You have about 3 seconds before he tells you what he wants.
When he goes "Divide by..."
Subtract your HP from the next highest divisible. That's all you need to do.
For example if he says "Divide by 5" and you have HP of 7... you need to go to 10. 10-7=3. Jump in the #3 circle. He gives you nearly 10 seconds to do this.
Surely you can work out a single digit number in ~13 seconds? You can do half of the calculation before he even tells you what to do. In fact, you can start on the calculations before he says what he wants:
Example: Your HP is 4.
Divide by 3? I need to step on 2 (4+2=6)
Divide by 4? Don't step on anything.
Divide by 5? I need to step on 1 (4+1=5)
Prime? I need to step on 3 (4+3=7)
I could probably do half of that before he tells me what he wants.
EDIT: Before anybody goes "B-B-BUT there's everything else going on!"
I watched the video. The battle comes to a halt, the enemy doesn't do anything during this time. You have all the time in the world to make your choice and step in the appropriate circle. Choosing the correct circle should take priority over your DPS rotation, and given he sets your HP max to this number, there's no healing to worry about either, and he just stands there while you pick your numbers, so tanks don't need to worry about their tanking either. They deliberately designed the battle so you can think about your answer. Obviously, the faster you can pump out the answer, the more you can DPS, esp if you're ranged.