I originally had my Zumo 550 on a Ram mount off the center of my Heli riser. Not a bad location, but I had to look down to see it and had to reach into the middle of the handlebars to make any adjustments. Ok while stopped, but not what I preferred while moving. btw: I mean MINOR adjustments like raising/lowering volume or skipping a song or such...not trying to do major routing or such that you should stop to do.
At Americade two years ago i saw another ST1300 that had an extra long Ram arm mounted on the left handlebar. This puts the Zumo higher, more in your view (higher tha the top of the dash) but off to the left side so it doesn't block the speedo/tach area. Easy very short reach of left hand to make adjustments and then the left side buttons are very useful. I greatly prefer this spot and when stopped I just put my full face helmet over it.
BTW, We all remember to secure the Zumo itself when the bike is unattended, but please also give some thought to the replacement cost of that Zumo mc mount....it ain't cheap! SO, what I did is replace the standard twist to tighten knob with the Ram locking knob. That way the zumo mount is locked to the bike.
The Ram arm I am using is about 6 inches or so long, it's the longest one Ram sells.
Now I use that center of the riser plate Ram mount for my Ecort radar detector.