The way I see it, as long as you can see the puddle when you're welding and you aren't seeing spots when you extinguish the arc, you're fine. Another way to think about it is, use as dark of a shade as you can while still having good visual of the puddle. I have a variable shade hood and honestly haven't looked at what shade I'm on in a few years. I'm probably around a shade 10 for most of my welds.
Edit: according to that chart, I should be around shade 11. I believe it. I lost the "shade indicator" that goes on the knob for my hood last month when I repainted it, but 11 sounds right.
