We are using a very simple method to design transformers
we are using CRGO core, and an equation 7/A yes SEVEN devided by Area
'A ' area of the bobbin in squre inches (L x B).
Straight away You can go to scrap market with a inch scale and calculator . Select a used CRGO core. then start design by measuring the length of middle part of the E core (as Length 'L") and assume a breadth 'B'. If bobbin for your design is not available don'r worry you can make it by using old PCBs or hylam sheet.
We are adopting this method for design of small 230/12-0-12 500 ma transformer to 2 KVA stepup transformers.
7/A is simply derived from transformer equation it is acurate for CRGO
example if bobbin internal ( Hole of bobbin) length 2" and breadth 3.5" area is 7 sq inch then turns per volt is ONE (1 turn/ volt)