It's depend of your shooting style. Which light shapers you've planed to use, and what is desired diaphragm value at given iso(50).. Did you consider the need of pilot light, because they have only 100W max. Also the freezing capabilities.. In data sheets they are rated as 1/1200 sec. You mast divide the value approximately by 3 for t.01 and will receive about 1/400. This is at full power, lower power values are with longest flash duration times. So 1/400 is almost ok for quite static objects and camera. Also I just called the local distributor which is friend of mine, and he told me 200J at full power with soft box like 66, should give about 4.0 / 4.5 at iso 100 and 2meters to the object. Also hi sent me a reference table just for a starting point.