1260 feet?
600' = 1020mb there is a drop in pressure of 22mb which equals a height of +660' [1mb change ~ 30'] so 600+660 = 1260....Assuming a lot of assumptions of course, like ISA temps and the like.
EA
Edited becasue I thought I made a major balls up. But in hind sight the 300' AMSL of airfield B is a red herring, all you need is the pressure settings as far as I can see....