Inside 30yds, most any decent rifle thats at least as powerful as most 10M rifles (around 550-600fps/8gr) will drop squirrels with a perfectly placed hit. I have killed lots of squirrels with my FWB90 air pistol with iron sights, but kept distances inside about 10 yards or so.
For offhand work, I'd say MOST shooters should stick to under 18yds with air rifles for squirrels if shooting offhand.
But if your gun isnt perfectly zero'd, and you are not able able to keep your shots inside a nickel at the distance you are shooting, you need to move in closer, to where you CAN hit nickel about every time, since more power doesn't usually help til you get up into the "rimfire with hollowpoint" level, and even then, a runaway is very common with a poor hit.