For me, on this picture, with a 'close' offhand or a good rest shot, bottom of the green dot, just behind the eye on the crease. A bit high will still catch the top of the brain, bit left, brain, bit low, brain and spine, bit right brain and spine. Any accurate bullet will work - I'd probably use my CZ .22 Hornet with a 50gr. Spire Point Hornady at 3,140fps - or a 40gr. Sierra HP at 3,240fps. (14.0gr. Lil'Gun in RP brass)