Mike
You don't say what size your studio is or what color the walls are. You could be getting some reflection if your walls are too light or too close.
However, 300 or 400 watt seconds is pretty bright. I shoot ISO 100 in the studio and shoot f5.6 or f8 -- depending on the set, results desired, etc.
I don't think you're readings are too out of line. Go to a slower film and -- if your studio is small or very bright, cover the walls or repaint the studio!
Regards
Bob
Something around f8 with white seamless.