yes there are loads of formulars for calculating transformers - we definately don't just guess! lol
the first thing you realise is that the typical example given in collage of a transformer having 2400t on the primary and 120t on the secondary doesn't make it a 240 to 12v transformer, as that formular ignores regulation (due mainly to resistance). It also ignores wire gauge completely so is the 1200t 0.1mm diamter or 2.0? even the number of turns is just a random number. a 3va transformer will have a lot more turns, a 1000va a lot less.
unless you know the answers to all those questions, and can then get hold of laminations, bobbins, wire etc - i wouldn't even bother.
how many volts are you after? at what power rating?