If you're talking about the plastic cap bolt - Shimano says .7-1.5 Nm. I just hand tighten with tool by feel, similar to how you'd tighten a headset cap. Take out the play and maybe a tighten a little more, as long as it doesn't bind.
The actual Crank arm fixing bolts are 12-14 Nm, which I believe is critical for accuracy if you are installing something like a Stages, 4iiii etc arm.

Use a little grease on cap and bolts but NO loctite!