Yes, head shooting a squirrel is a challenge.
My perfered shot is in the side of head between the eye and the ear. About 1.5 cm in size.
My rifle is sighted at 50 yards.
I use trees as a rest whenever possible but years of position rifle shooting has served me well while hunting.
My ammo is SK Match which I modify to a deep and wide hollowpoint using a special die/shellholder I made to do this in a reloading press.