To value and hedge your short position on a binary option, you could approximate it using vanilla options and the Black Scholes Model.
You need a fixed payoff amount and not a payoff relative to the spot value, so you could buy an appropriate amount of a call spread with a very small difference in strikes.
Let's say you buy 300 calls @ 859 and sell 300 calls @ 860. Effectively, if the stock turns out to be over 860, you will have the 300 dollars to pay off your short position.
The value of the binary option should not be too diferent from this call spread, so you can value these two vanilla options with the Black Scholes model to get a rough idea.
This is an approximation with tradeable instruments, and it's not exactly the same thing. For one, your hedge is better for you than your short position so it's natural for it to be more expensive (imagine if the stock finishes at 859.5) but hopefully it's usefull to get the rationale.
Theoreticaly, you can of course just use the N(d2) from the Black Scholes formula to get the value of the binary.