i would like to put a track for the Staysail, on the side deck, on our W42 ketch. Has anyone done a similar addition, if so where? My research so far is the 10-18degrees, which puts the track midway across the deck. Would this improve the “slot” to give our W42 improved sailing

Our 42 has staysail tracks on the main cabin top just forward of the dodger.  We find it does help going upwind to have a tight sheeted staysail. This layout also lets us heave to with the upwind sheet tight (and run inside the shroud) with no chafe issue - the clew is just forward of the mast at that point with our sail. Going downwind we sometimes change the sheets to run between the stays for better sail control & shape.  Overall this arrangement was great for high wind/heavy weather when the staysail would become the sole sail in use. We have a heavy duty staysail on a furler that doubles as a storm sail.