I use an old 545 holder for Polaroids. A lot of professionals use Polaroids as a proof before shooting the regular film, and swear by it. The thing to do is use a film and Polaroid of the same speed, or as close as possible. An example would be using Polaroid 54 to proof a shot in TMax 100. Both are nominally rated at ASA/ISO 100 speed. Composition can be tricky until you get used to it... The Polaroid will be smaller than the sheet film. Just compose for film, using the entire groundglass, and then take the Polaroid proof to check exposure and lighting ratios. Polaroids are a great learning tool also, because you see the results in a minute or two. Any mistakes become painfully obvious and can be corrected on the spot.