My approach is still adapting, but currently I have been using an approach to systems based on percentage of reactor used (i.e. looking towards the final performance experience will be for the pilot).
So if I'm working with a L10 reactor on a fighter, I am assuming maximum mobility at whatever power cost first and foremost (though I don't use mobility chambers for my drone fighters as of yet because I'm not sure whether they are of yet any use to the AI).
Next I add shields based on the fighter's role: if it's a Superiority fighter, then I aim for shields that occupy ~25% of the reactor's output when full, and ~50% when charging; if it's an Assault or Shock variant, then I aim for shield profiles more like ~20%/~35% or ~35%/~65%, respectively, because those shield profiles suit the way I want to use that model (i.e. fast, disposable assault unit with high DPS, or heavy shock unit meant to endure more punishment at the cost of DPS - both are specialized compared to my general superiority fighters but in different ways). So I add as many blocks get it to where I want it in terms of performance.
For a fighter, weapons are really the whole rest of the power loadout. I choose my one or two weapons for it, and grow them until they use up
ALL of the remaining power potential - with perhaps a tiny bit of over-draft if thrust is engaged - when firing.
When I know AI is using chambers, I will probably add mobility & stealth chambers on my Assault craft, and shield defense chambers on my Shock craft (improved shields + alpha reduction), probably a reactor overclock chamber and stealth for my Superiority fighters.
So I guess the number of blocks I use at the moment is not a function of a set formula - the numbers have been changing so much it's a waste of time to calculate it first - but rather a rough slice of the total power pie based on role considerations and established by saving and testing it at each stage of development.
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I allocate 20-30% of my power to shields just so I can take some stray hits, but rely more on maneuverability and/or armor when it comes to winning a fight.
For weapons, I normally aim for 40-60% power consumption.
A familiar-sounding approach!