Did you try and use their PI Expert software?....it does the transformer design for you.
Anyway, you should just check that core does not saturate with peak primary current.
Also, that core losses are not too high.
Also, you need to be sure you are not reflecting too much voltage to the primary, otherwise mosfet will blow.
Also, you need to make sure you do not reflect too much voltage to the secondary, otherwise diode will blow.
Also, you must have gap for flyback.
Also, you should have proper mains isolation, so use triple insulated wire or enamelled copper wire and margin tape to keep good clearance distance.
Remeber that if you have a too high NS/NP ratio, then your duty cycle will get too small, and your on time will go below the controllers minimum on time.
But if you have too low NS/NP ratio then your duty cycle will get too high, and you will be cramming the secondary conduction (the highest current one) into too small a time and secondary rms will be higher than wanted.
Also, the inductances must be high enough to keep you out of too deep discontinuous conduction mode...but DCM is a good mode...just dont go too deep into it else peak current is too high.
It is nice for you to do DCM...because you dont get noisy sec diode turn off.