Thermal Design advice needed

I have a bank of 4 of these:
http://www.irf.com/product-info/datasheets/data/irfp3206pbf.pdf

Bolted onto a plate of 4" x 6" x 3/8 "aluminum, with a 120mm,12V Fan cooling.

The load thru them takes them to about 85C (case temp), a 50C delta. Peak 800A pulse currents (shared by 4 FETS) at < 50uS, 2.5% DC.

I was just wondering how to calc. the max safe operating 'case' temp for these units given these conditions.

thanks...

What is your calculation? Keep in mind SOA includes max current for avalanche diode appears to be <70mJ @ 85'C Tj , but you measured Tc, so factor Eas*ZthJC to get rise.