This seems like the idea of mining shares sold on bitcoin stock exchanges. They pay you daily, weekly or bi-weekly with a calculation like yours. But probably the provider do this for more profit than %7.5 . If you can find trustable people you can earn more with this method, but it's hard to trust people on internet and even if you find the people, it's not easy to buy good miners for a suitable price if you don't want to wait. If you wait then the difficulty will increase on and on. Only if you find a person with a mining rig who wants to sell shares of his system, this will be a good idea. And when many people are involved what will you do when miner is broken, will everybody still trust the provider, i don't think so. I think that's why the stock system is being used.