They say Below the horizontal plain of the wheel cyl. or caliper, r-drum or f-caliper or any combination of the two, but some MC's had them built in to the rear brake circuit, way higher than the H-plain so.....
Also as close to the MC as possible.
Don't know of any reason why you couldn't mount it vertical, but I'm not sure, I would probably mount it horizontal.
The rear proportioning valve gets mounted in-between the RPV and wheel cyl./caliper.
Ck this out.