Grit means very little for sharpness beyond ~2000. Far more important is a consistent angle.
This is a good article that explains the angles
a good sharpening tool will have a angle guide, something like this
I'd recommend against "eyeballing" it the first few times.
Joerg Sprave made a nice video explaining a lot, he uses the Gatco set I linked.
There is no set way of knowing exactly what grit to use but you can estimate. <1000 grit is for repair, so I would suggest visual inspecting the blades edge under a light. If there are any points that reflect light differently, or that you can see any damage, then use the 400 grit until the edge is consistent.
If your knife passes the visual check, but ...