You can measure the primary leakage inductances on your push pull transformer, you can assume the dominant cap will be the mosfet, use a snubber cap 5-10x this value.
calculate R from 0.5 x SQRT(L/C), if you have very high leakage and/or reflected energy from the secondary there are other ways to work out the RC based on Ipk at mosfet turn off and max power you can tolerate in the snubber R (e.g. 3W may be practical) this allows a higher value of C and more effective snubbing. Regards, Anna.