What fishing method will you be using it for the most? Handlining, pitching jigs, contour trolling while steering with the kicker? If you only use it for trolling and steering with a bow mount, directly in front of the main motor is fine. If you plan to stand on the rear casting platform and pitch/cast anything, I’d either rail/track mount it on the gunwale.
http://www.in-depthoutdoors.com/community/forums/topic/my-new-wx2190/
Check out this post, I’d mount is about where this one is, but on the other side(personal preference because I try to cast off the Port side).