Question 389460
If the side length = 18.5, the diameter (from flat to flat) = 44.66"
If the diameter = 44, the side length = 18.225"
That might be more accuracy than you need, since it's probably made of wood.
Anyway, using 44 for the diameter and 18.225 for the sides:
Adding 3" makes the outside side length 20.71 inches.
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s = (22 + 3)*tan(22.5º)*2
s =~ 20.71