The way I see it, I sure hope that there is no optimum shield:weapon
or recipe, and I don't think there is unless you lock down exactly what opposition you are facing. Everything about SE4 is about action & reaction. Feint & counter, advantage & disadvantage, etc. If you see your opponent's designs and don't react, you will probably be overrun.
The other part of the question, mentioned by others, is how the ships are used. I make different designs for offense, defense, and support. These are further different on how or where they operate:
how near to resupply? --- determines how much supply, solar collectors and/or quantum reactor used.
solo or in fleet? --- determines amount of PD, supply, and to some extent, strategy assigned. Also is affected by stage of game since fleets are more common later in game.
suicide mission? --- different design from a ship that I plan to keep around.
expected foe: planet? fleet? each affects design.
desired result: enemy destroyed? enemy captured? enemy delayed? enemy blockaded?
Which way is the front moving? Am I advancing or retreating? What is the urgency of attack? How does my economy support my military and compare to the enemy economy? Am I ahead or behind on research or intel?
All of these things affect my designs to some extent.
Maybe I just worry about too many things and should go all weapons.
Slick.