Bpk = E / 4 F N Ae, ( square wave ) for 50kHz, a conservative Bpk = 150mT peak each way, the core area = 230 mm^2
Thus for 340V out Ns = 50 turns, except the duty cycle must be taken into account so divide by 0.94 = 54 turns
10.5V is Vin min - so to get 340V ave out @ 94% applied square wave, Np/Ns = 10.5/340, giving Np = 1.66 Turns
Obviously we round this up to 2.00 turns ( + 2.00 turns for the other ) for the pri, giving 69T for the sec
[ check 10.5V * 69/2 = 362, * 0.94 = 340.5, maybe add a wee bit for the diodes and the wiring loss in the Tx ]
The aux is therefore 25/10.5 * 2T / 0.94 = 5 Turns