Torque isn't terribly important on bolts like those, but here's a simple procedure. Tighten the bolt until all the looseness is taken up (no gap visible between the bolt and the cover) then turn the bolt head 1/6 to 1/4 turn tighter to compress the gasket slightly and prevent the bolt from loosening. Overtightening a bolt on a sheet metal cover can distort the cover and/or damage the gasket. You could also use a drop of a thread locker such as Loctite (blue color, not red) on the threads.

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