Here are the formulas you need. Use whatever units you want for your measurements. Let

a = diameter of the upper circle of the frustrum

b = diameter of the lower circle

h = vertical thickness of the frustrum.

R = the larger radius of your flat arc.

r = the smaller radius of your flat arc.

$\beta$ = the central angle for your arcs, in degrees.

Then the calculations you need to get exact answers are:

$R = \frac{b}{2(b-a)}\sqrt{4h^2+(b-a)^2}$

$r =\frac {aR}{b}$

$\beta = \frac {180b}{R}$