Not Quite
Sing,
U would divide it by 320 if u want a 3.2 deg glide slope path, it works like this if u want to maintain a 3 deg slope divide the height to loose by 300, if u want to maintain a 3.5 deg or 4 deg slope you would divide it by 350 or 400 resp to give u the distance and so on
U mentined the Jepps, these days in all the approaches in the profile view everything is there (the angle the distance etc)
Bare in mind there will be minor adjustments if reqd to stay on profile by either inc or dec FPA or Vspeed. The above formula if u like will enable u to quickly calculate u'r profile altitude for a given distance mentally, thus can also avoid chasing it and doing what I call the snake dance
Ofcourse on the 320 u don't have to waste u'r mental energy so much (no that its hard) cas u have the brick which indicates u'r VDev and is pretty much like a glide slope indicator
yes. Adding the .3nm, is a rule of thumb. Like any aeroplane u always have to be ahead of the aeroplane this rule of thumb in this case helps do exactly that as IDG mentioned it a takes a lil time from the time u command it for it to happen, same with the turns, from the time u initiate the roll command it takes a lil time for the aerolplane to turn, so if u do not anticipate it in this case u do the horizontal snake dance.
Hope it helps
Ta