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:

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

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

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