help with building a transformer inverter

[kosacid]

ok i got hold of a old transformer and im using 2 irf740 and a arduino for switching using a centre tap, it is a variable frequency pwm
that is all working fine the problem i have is it puts out the right ac voltage but no load so i ended up pulling the transformer apart
found out it only had 10 turns and 40, i have looked over the net and i cant work it out how to calculate the centre tap so can someone show me
this is what i have
the centre tap was 1mm copper wire
the core is 20mm x 20mm depth 30mm
i didn't unwind the primary but it is 68ohm 45.0mh 0.25mm copper wire
the iron core is 65mm x 55mm
so can some one tell me how much winds it needs and show me how to do it, it will be running of 12v
and what difference will it make if i wind it with say 0.5mm copper wire with a centre tap