I would suggest you to buy something like 5 bitcoins when the Bitcoin price is 280 USD, and sell 4 of the coins when the price is around 350 USD,
5 X 280 = 1400
4 X 350 = 1400
so you will get 1 btc free of cost.
Your calculations are fine so far.
The only thing you have to do right now it to wait for the price to decrease.
But how do you know it will go down that much?
I dont want you to stick to the prices that I have mentioned, just buy it when you are able to and sell it at a good price and earn like 0.5 to 1 btc as a profit.