Hey buddy just do simple calculation. check the cpm rate of your advertising network. For example imagine the cpm rate of one banner is 0.1usd ok . now go to preev.com . and check 0.1 USD is how much BTC. now divide 0.1 with 1000. it comes around 0.0001 USD or 11 satoshi.This is what you get from ad network if someone comes and claim from your faucet. suppose imagine you keep the reward 100 satoshis . That means you will give each visitor 100 satoshi and in return you will get 11 satoshis from the adnetwork. If you place nearly 11 banners of 0.1 usd cpm you will be able to fix reward 100 satoshi. So while fixing the rewards first calculate cpm of your adnetworks and fix the reward accordingly. Also keep in mind that you should recover your hosting expenses also . Hope this will help you to fix the reward.