I sight my Remington 541-T for 50 yards...When I used my Remington 552 I sighted in for 25...
With the 552, I could keep all my shots inside a nickle at 25 yards...My 541-T will do that at 50 yards...

Now, I used the 552 from the time I was 16 (1971) until 2000 and killed a ton of squirrels with it...

As mentioned, 50 yards to head shoot a squirrel is a good ways, using hunting positions...I can do it with my 541-T but wouldn't have tried with the 552...

Let your gun's grouping decide how far you should be taking those shots...