I realy confused about transformer calculating !

I have a ETD39 core and I like to build a smps that 12v to 220v with push pull topology...

I have got this questions:

1.What is Bmax in push pull topology?

2.What is maximum power outpout for ETD39 on 100k and 150k?

3.How can I calculate second question? what is the formula?

Thanks