When I took over a gym, I wanted to level it up. I battled it with my most powerful Pokemon (who was about twice the CP of the pokemon in the gym) and I gave less prestige when I used a more powerful Pokemon (+100) than when I used one closer to the CP of the gym'd Pokemon (+284).
So, how is prestige gain computed? Prestige loss is fairly static but it's unclear to me what the rate of gain is and what influences it.