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Never used one. The starrett one is handy because it gives you a very accurate read out of the angle you need to cut to make it work. As far as I know pro coving guys put it up with a suitable type of plaster. Never done the job or had it done or seen it done though. I like coving but not the ordinary stuff, a nice big complicated coving really sets improves some rooms.
You could fix the edges of your ceiling for less time/money though.
These jobbies any good?