Transformer loss = R(ac) x Irms primary plus same for secondary, plus core losses best estimated from data sheet for cores being used, e.g. 100mT peak at 300kHz, most good materials will have losses of around 200-400mW per cubic cm of material.
Add all 3 together, hope that you are using 3F35 or better core material (or < 50mT pk for 333kHz) and away you go...
p.s. note that R(ac) is the resistance of the wire at 333kHz, often several times higher than Rdc, unless you are using 44-48 awg stranded litz wire, and there are no big gaps in the ferrite near your first layer of winding....
good luck...