There couldn't be any sureshot formula for day trading as you can't be sure about human behavior. The same person reacts differently to the same situation at other time. However, for day trade, you can guess how the stock market will behave to a given situation and accordingly you have to trade.
My strategy for day trade is -- Going with the market news during night & in the morning I decide to trade in two or three stocks [either to go long or short for trade]. I go for 25% of the available fund at the begining. If the trade turns out to be profitable, then with a decided target, I book the profit at the earliest. If not, then I average it TWICE with 25% of the balance fund. If still the situation worsons, then I start booking possible minimum loss in remaing period.