In order to harvest you need a vested balance of at least 10,000. There is no real way to calculate profit because harvesting is sort of a luck game and it is based on your wallet importance level. Some blocks harvested will have fees included, which you get to keep. Other will have a zero balance. I have about 25k vested balance right now and have been harvesting for about a week and have only harvested two blocks. One had a little over 17 XEM and the other had zero.