Dani is correct: 150 fans on = 300 fans off approx. Carbon brakes work best when they're warmed up to a reasonable temp, not when they're cold. Different brake manufacturers give slightly different advice on the use of brakes and fans. Just do what your company SOPs say as they will have been set up with input from the brake manufacturer. Also, different airlines may have different cost structures so may decide to use/not use the brakes and reverse differently. For example, some use normal reverse with manual braking below 100 kts while others use autobrake and idle reverse (runway length and condition permitting) to get the same landing performance. Turnaround times also come into the equation. It's always best not to put the fans on for at least 5 mins to allow the brakes to thermally equalize. Personally, if I were writing the SOPs I'd leave them OFF until pushback on the next sector unless the temp was very high. The limiting factor appears to be the flash point of the hydraulic fuel in case of a leak.