design of push pull converter transformer

I need a little help on designing a transformer for my push-pull converter.
The input voltage is from 9 to 18v,output should be 350v,operating frequency 100KHz,output current 0.2A and the core should be a ferrite one.
The push-pull is a center tapped version.
What value should i use for the flux?
Any help is welcomed.

Normally for the Ferrete core flux density will be 0.3 Tesla

please check from Ferrte core datasheet for 100kHz - this will be 0.15 T to 0.2T - choose in between Value - according to this core loss will vary.

with this calculate Ap = Aw x Ac area product.

choose core