butting backer board up to plastic shower pan
The bullnose is to finish the sides. Your backer needs to go over the lip of the pan, within 1/8" or 1/4" of the lower lip on the pan. Your regular drywall will be attached directly to the studs throughout the bathroom. That will get you TO the top lip of the pan. Add your backer over the drywall. You will need a bullnose to finish the edge. You can "strip" the studs in the actual shower area as wrangler suggested to save buck or two....
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