AS mark is /300
A2 mark is /600
So add 300 to your AS mark to get your maximum A2 mark.
Instead of adding 300 to your AS mark, try doubling it instead, and then maybe taking a few marks off (because most people do worse at A2, apparently. I know I did!).
Or you could just do what I did and think of the grade you're realistically aiming for at A-level, find out how many points you need to get it and take off your AS points. For example, if you wanted a B in something you'd need 420. If you got 215 at AS, 420-215 is 205, so that's what you'd need at A2 to get a B.