How to calculate or design glide ratio?
One simple way is to compare thrust requirement for level flight at Vbg, where lift to drag ratio is optimal, to the weight of the plane.  This ratio of pounds of thrust, which is equal to pounds of drag in steady state level flight, to the weight of the plane should give a good approximation of the glide ratio.  Glide ratio is how much altitude do I give up to move how far forward.  The altitude is the power source.  How far forward you move is dependent on drag.

So an 1800 lb Cessna would require 200 lbs of "thrust" to glide 9:1.

Now to design for it, speed is a critical factor in selection of wing type and loading.  This is the albatross vs eagle consideration.  Higher speed cruisers favor the albatross.