for exposure (not composition) checks, I use 'standard' FP100C (not '45') in a Polaroid 405 back. I'd like to have a go with the 4x5 instant film, but the 550 backs still seem to be not that cheap... I'd love to find a $40 550 !!
(Fuji FP100C45 fits the Polaroid 550 or Fuji PA-45 back; Fuji FP100C fits the Polaroid 405 back)