If you are thinking of "day trading" BTC then this is a simple guideline that might be of some use (and as the title points out don't risk losing money you can't afford to lose by doing "day trading" as it is inherently a risky activity).
Firstly - do a thorough check about the exchange you intend to use for your day trading (make sure they are established and that they can prove they have BTC reserves in cold storage and after that consider things like their trading and withdrawal fees, etc.).
Next - only inject as much actual fiat as you are prepared to lose (i.e. think of it as gambling and that you are taking a bet).
Next - assuming that you are starting with fiat you need to purchase some BTC with that fiat. In order to give yourself the ability to buy at a potentially lower price later you might want to simply purchase BTC with half of the fiat that you injected (so now you have half fiat and half BTC held in your exchange account).
Next - create bids and asks and record them yourself (don't just rely upon the exchanges records and be sure to check they are reporting everything accurately).
Next - if you manage a buy then create a sell for a price greater than you just bought for (allowing for any fees of course) and if you manage a sell then create a buy for a price less than you sold for.
Hint - it is always worth having a very low buy and very high sell in place just in case of dramatic price changes.
Next - do not deviate from the method - if you end up running out of fiat or BTC then you just "have to wait" for the price to enable you to trade again.
The last point is perhaps the most important - if you can't stop yourself from breaking such a simple set of rules then you are most likely going to end up losing.
Can you profit out of such a simple day-trading system?
Provided that BTC remains as volatile as it has for the last few years then it is very likely that you can.
it is really a very good calculation you have provided , i was thinking of going for trading bitcoin, actually i was just buying bitcoin from exchange and sell them, this time i will try your formula with fiat currency and check how much can i earn.