That's a nice diagram. The only thing I would change is where the 20 amp main fuse is. I run the red from the reg/rec straight to the battery, then have the fuse between the red wire between the reg/rec and battery and switch.
The main fuse should be between the sources of power and the rest of the electrical system. In most cases there are two sources of power, the battery and the alternator. Lets say something shorted and the main fuse blew. If there is enough residual magnetism in the rotor, the alternator will keep putting out power, this can keep power flowing to the short, making the short worse and causing a fire.
I also ran the brown to the reg/rec and brushes from the fuse box with a 10 amp fuse.
Your horn is still not grounded right. The ground comes off the horn button. Power in on one side, out to the button, button to ground.
Leo