Here is a calculator that might help in making truncated cones (frustums).
Cone and truncated cone
If you can't find a mathematical formula to find the length of the arcs some string and a ruler always helps. That is what I use most of the time. 
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~Doug~
AC010505 EAMUS CATULI! Audere est Facere THFC 19**-20** R.I.P. it up, Tear it up, Have a Ball
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