Pick your shots carefully, so that if you are having to shoot up into a tree, there is a substantial tree trunk or limb behind the head of the squirrel to stop your ball. Use the light loads. 27-30 grains of FFFg powder should be more than enough. If you can't get a head shot, then aim for the front shoulders. there is not a lot of meat there, and losing it won't be so painful.
Around here, tree shots are best left to a shotgun using nothing larger than #6 shot. Its not uncommon for two hunters to hunt together, one with a rifle, and the other with a shotgun, so that there is always a choice available to take a squirrel in a tree. Most shots at squirrels will be less than 25 yards, from muzzle to tree rat. :hatsoff: