I am trying to recall a formula for calculating correct up/down price
of a certain commodity at any given exchange at any given time. Seen
it somewhere a couple of years back and roughly remember a set of
arguments, which include:
 
1. current price of the commodity
2. price at the end of the last hour
3. volatility of the market
4. time remaining to expiration
5. the commodity futures price

Now, my question is, what could possibly be the function, based on these?