I think that you related the pressure to the volume incorrectly.

An indirect proportion, such as pressure x volume, follows the relationship that when one of the values is increased, the other value must be decreased to keep the product of the two values the same. Think of the example 3 x 4 = 12. If you increase the 3 to 6, what do you have to do to the 4 to make the product still equal to 12? You have to change the 4 to 2. You doubled the 3 to make 6 and so you must multiply the 4 by 1/2.