compound cuts/tapered staves query

Folks -
I am making small planter for my niece - she wanted a round one. I went to
to use their online compound angle calculator and something looks amiss. As an example, if you have a vessel with 12 sides and a 12 degree taper then each stave takes a 30 degree bevel, split, so it should be 15 degrees per side, no? Why does the calculator show a discrepancy of 18 seconds?
I did an 11 sided vessel at 11 degrees for my niece ( there's an old Grateful Dead song called "The Eleven" ) - the fit for the bevels wasn't quite right, but I attribute that to not being able to go to tenths of a degree on my miter guage or bevel on the saw. It glued up okay, but I'd like to figure out a way to reduce the compound error in not being able to set TS angles to the nth degree.
Ripping the tapered staves was a pain in the ass. My taper jig is only good for one side of the cuts, so I'd also like to know if any of you have done this, and how. My kindling box just got a significant fresh infusion.
John Moorhead Lakeport, CA

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