I've used an Accuset 1000 for 10+ years for poly plates.
Plate material from AB Dick/Presstek/Marc Anthony - Item 66534, Megapro-LD 5 mil poly, Spec 820, 13.3 in x 200 ft rolls. (These machines are not spec'd to cut thicker material.)
I cut with a Dahle Cut Cat mat cutter. Roll up the plate run out of the processor. Sit it between myself and the cutter. Unroll into and under the blade to the proper point. Slide the cutter device. It's an eyeball technique.
The imagesetter has a Center Capstan feature which gives me about a 1 inch unexposed area, the width of the plate. Without center capstan on I think there would be no way of knowing where each plate starts/stops. I was probably lucky years ago when we delved into this initially. I cut to this area. (Actually there is a plastic guide I align the tail to leaving .1in silver on the plate)
I plate everything from InDesign - InDesign's ability to create Print Presets is great for this. Plate size 13.3 x 19 (AB Dick 13.3 x 19.3) - 1 inch offset (for my 3302; 1.5 inch for AB Dick). ID can rotate the image (I use for envelopes and some flat sheets) during plating, has full lpi, screen angles, separations, ink aliasing and trapping. (Because of the 5 mil plate I also scale 99.5 in the direction of travel through the press for proper image length.)
For accuracy you should use InDesign w/ crops for accurate plate sizes. The Center Capstan function returns the capstan roller to the exact same rotational start position for each plate/page. This is to compensate for any variation or wobble in the capstan roller and will slightly increase accuracy of registration. Therefore, dependent on plate/page size, you could get from zero to over three inch gap (nearly a complete rotation). Imaging with crop marks will get you very close to actual desired size and as mentioned above you can scale it to get dead on accuracy.