Magnetics Voodoo ,nah ,its a science alright ,fairly complicated subject in its own right ,theres many different coil configurations ,then you have various different core materials ,shapes and sizes with different properties for different jobs . Transformer designer is more or less its own speciality . CJ who posts here is very knowledgeable on the subject ,he's made great posts of transformer teardowns ,where he takes a damaged transformer ,carefully unwinds it ,noting the wire gauge used and the no. of turns ,then rebuilds it to factory spec.