Barry, I don't think I am clear on your question. Studs either run 16" or 24" on center. Either is accepted by 4 x 8 sheets of drywall. The perimeter of the ceiling "should" (smile) have nailers. So either whole or cut sheets should span the studs and nailers. Otherwise how could the existing sheets have adhered to the ceiling? What do you mean " cut the drywall in half"? As you tear down the existing ceiling you should be able to see how the ceiling was originally patched together and replicate. Also has moisture warped the ceiling? Maybe use green board so the problem doesn't recur?