Instant film on Mamiya RB67

I got a Polaroid back for my RB67 off of ebay. I had already purchased 3 boxes of Fuji FP-100b instant film. I loaded a pack of film and set up the camera. I was photographing inside, using 4 60W fluorescent bulbs and umbrella to light the subject. I set the camera at f3.8 and 1/8 sec shutter and fired away. First shot was a little overexposed. I decreased the shutter by 1 stop and took another shot. A little underexposed this time. Nonetheless, the images were usable. On the RB67, you get a square image. You don't make use of the whole film. But it's fun and interesting. I just ordered some Fuji FP-100c color instant film. I'll be testing this soon.