If you bet correctly then you will double your BTC each day.
It might make sense to actually put that on the site, no?
For your cutoff time, is that the timestamp of the block that the transaction is included in?
It appears you are pricing the target at some level that varies to some degree from the previous close. For instance, the FB is priced about 1.5% above the Friday close. The cutoff time is after the market has already been open for a half hour. A speculator would be self-imposing a handicap by wagering before the market opens. But the target price is made well in advance. As a result you are likely to have large amounts of bets on one side, causing an imbalance. I presume you do not plan to always take the other side, so how to you balance the bet / hedge?
Specifying that you have a 0.5 BTC limit is ineffective. WIth a digital currency that can be used anonymously, you have no idea how much wagering a single party has made through multiple 0.5 wagers. As a result, you have no limit as to how much wagering occurs.
This really seems to be a system that can be gamed. The only unknown is how much in losses the house will actually pay out.
Perhaps you might wish to refine your bucket shop concept before going live.