Yeah so it works as a percentage, not as a whole like on most servers. Most servers it works on a number system for example: If a drop rate on a monster is 1/300, and my drop rate is 150 then that drop becomes 1/150.
However, this is not a very balanced way of doing it as it only promotes P2P end-game players who can buy the highest drop rate. How it works on CustomX is as a percentage. Lets take those same numbers a 1/300 drop and a 150 drop rate. The drop rate of the item is divided by the percentage of your drop rate that it would be affected. In this case, it would be about a 1/20 drop (300/15 = 20).
And that is how it works a bit complicated to explain but a far better system for overall balance
Also see Double Drop Rate