Lemme see if I can 'divine' it out...
One gallon of 6% bleach is 0.06 gallon of actual chlorine.

In 18000 gallons, 0.06 gallons makes a ratio of 18000:0.06, or 18,000,000:6 or 300,000:1 or 3,000,000 to 10, which is 1,000,000 to 3.333. That's 3.3 parts chlorine per million parts of water. Whew.

So 1 gallon of 6% bleach should add about 3.3 ppm to your pool.
25ppm divided by 3.3 ppm per gallon = 7.5 gallons

So yes, you will need about 7.5 gallons to achieve 25 ppm.