I don't really understand the colour section of that graph. For me, it would not be a problem to brew a beer of this colour with these salt levels. I'll look into this more in a sec. Just have to go out.
For the Mg, I would be aiming for 15ppm as I said above.
Edit: So I used your nonograph and plotted the beer I made on the weekend. It came out just above 5.6pH, at -50 residual alkalinity. I was aiming for ~9EBC. Not sure what to make of it, but just thought another example might help. Somehow.