Note: The math is a little more complicated if it's not actually one-half of the season left.
Then it would be Second Half Avg * Games Left + Current AVG * Games Played / 162 = Ending Average.
Your Ending Average minus your Current Average would give you how many points you can move up.
I think the Math is a bit more complicated than that, since it also depends on ABs. Lets say you are in a deep auction keeper league, and because of the depth, at the beginning of the season everyone has to field some number of platoon or bench players. After a half a year, your AB total includes those guys that aren't getting regular playing time.
Being a keeper league, you complete a bunch of trades sending great low dollar keepers to get back high priced or expiring contracts for players who hit with higer average. But your roster has changed so you have fewer (or none) platoon and bench players. In theory, you should get more ABs in the second half of the season than you did in the first. So your potential gain might be more than 1/2 the difference between your average in the first half of the season and the average in the second half of the season.