Just relating how I think about fleets when designing them, and curious as to how others do so as well.

1.)  Fluff Side:  I see pretty pictures in my head, I imagine what sort of race built the ships in those pretty pictures, I imagine how the weapons interact with their opponents, and with the ships themselves.  I think about how the people that built them think, and what their likely opponents are (No navy exists in a vaccum).

2.)  Conversion.  Build a fleet that does, in SM, as close as possible to what the fleet 'in the head' did fluffside.  Not always a perfect 1-1 correlation, but no created thing is perfect.

3.)  Crunchanalysis:  SMX, ultimately, is a game, and we are wargamers.  Some things work and some things dont, and some things may have come out of that conversion that wont work, no matter how cool they appear in the head.  Those things get patched up at this stage.

4.)  Evaluation:  General evaluation critera:

a.)  How does this navy deal with standoff issues, the longer-ranged foe that attempts to hold the range open?  (Usual answer is spinals, equally long ranged guns, or superior speed)

b.)  How does this navy deal with 'crushers', the fast foe with utterly lethal close range weaponry?  (Again, go fast, have massive point blank firepower yourself, or have at least sufficient firepower to attrit incoming vessels before they get to point blank where the ranged-based x, y, and z guns do you in)

3.)  Anti-Fighter issues:  How well does the ship stand up to its points value in FTL fighters?  I try to aim for fighting about even, here, because in an actual game, a ship may well get jumped by more than its CR/60 squadrons of fighters, and other ships may not be able to support.

4.)  Firepower vs. Surviviablity:  How quickly does the ship disable its identical twin?  If faced with its identical twin, would it have to go erratic to avoid being crippled?  Here, I try to aim for a single ship being able to shake off a single turns unreturned volleys in its longer range bands without meaningful damage, even without going erratic.  Going erratic should reduce incoming damage to the point where several turns of unreturned fire will be necessary to greatly impede the ships capabilities.

5.)  How gamist is my work?
ROF is cheaper, and better, than PEN or DMG.  Did I make everything ROF 2 or 3, and only then if then raise damage?
What about expendables?  Are there enough to wipe out whole fleets in 1 barrage at R18?  How about massively innacurate, and thus cheap, expendables with TDAR?
Stealth and sniperguns? 
None of these are necessarily immoral, and certainly not exhaustive... but its pretty clear when you look at your navy if the pretty picture in your head has been... influenced... by the hard realities of the system.
I also look at weapons and system fits here.. do I have the perfect damage track, with just enough speed, firepower, sheilds, etc. to minimize the systems lost to any point of damage?

6.)  Fun:  Do I want to play with this navy?  Would it be neat and interesting on the table, and does the design lend itself to interesting tactics?  Will it generate alot of decision points for me when I use them, such that victory or defeat turns on those decisions, rather than on the mathmatics of how MY navy interacts with someone elses navy?  And perhaps most importantly, if my buddy showed up with this fleet, would I scream in anguish, scream in joy, or go 'ooohh, that looks interesting, lets see how this nut can be cracked'

Just curious as to how everyone else thought/approached it.