yep, why would you substract anything - the derating limit is for the transmission, and the derating chart tells us at what MAP we reach that limit. Obviously the people who made that chart knew that there was a fan and belts between engine and transmission, so i would think they considered this when making the chart. They did the substracting for us (probably by simply measuring power at the transmission driveshaft instead of the crankshaft)
Also, 18% or 25hp or 19 kilowatts for the cooling fan? That would be one hell of a fan! 18% sounds more like what the T/R uses.