Is it a computer fan? If it is, it should have a sticker on it telling the voltage and the amps. You might need something a little stronger than a phone charger to make it work properly, although most fans don't use many amps. How big is the fan?
30% - 40% should be okay to start with. You will need to candle your eggs at day 7, and again at day 14. This will tell you how you need to adjust the humidity for days 14 - 18. If the air cells are too small, you will have to run lower humidity. If the air cells are too big, you will have to run higher humidity.