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I highly recommend printing the images out on regular paper first to make sure the scale is correct. I have made many boards and have had many times where scaling occurred - sometimes caught before etching and sometimes afterwards. Don't waste time and materials! Adding a border to the images in double sided boards will help a lot in alignment. Printing should be done on the darkest setting possible to get a good layer of plastic on the board and avoid broken traces. Also, if you can get ahold of a laminator (I use a modded AL13P) it is much easier to transfer a complete image with even heat and pressure.
I have done it many times and it creates very accurate alignment. I use overhead projector film to transfer and use masking tape to hold it in place, place the board in a sheet of paper folded in half and trim it so there is still some overhang to grab onto when the board gets really hot.
Circuit Board Design Class