Air Arms Owners Club

Could someone tell me the size of that little Allen/Hex wrench I need to use to tighten the small set screw that tightens the power adjustment knob on an S510 (this is the one on the bottom of the action when you remove the stock) and how much do I need to tighten it.

I've heard to just "barely snug it up". Over doing it can cause air leaks because of flexing? is this correct info?

Thank you all