Well, you do not have to apologize. This one is more trickey than most but this is the typical calculation for a truncated cone.
Volume cubic inches
pi × h × [(R²+r²+R×r) ⁄ 3]
= 1,549.85
Where
pi = 3.14159
H = your height to the top of the truncation
R = the radius of the base circle (diagram labels it r2)
r = the radius of the smaller circle
A cone when drawn in a drafting view where the front view looks like a triangle and the top view looks like a circle or circles as in your case where you don’t have a full cone. The length of the arc or arcs of the flat layout will be the true length lines in the front view those sides of the triangle.
Now you have to figure out how many degrees of that big circle you will use to get your cone. You start with your 16” dia x pi (3.1416)= 50.265 The true length line in your case is 40.797R x 2 x pi (3.1416)= 256.328. You can then make a ratio knowing there are 360 degrees in a circle 40.797/256.328 = X/360 solve for X and you get 57.3 degrees.
So take a piece of paper and draw a circle with a radius of 40.79 and 30.59 now draw two lines from the center point out that are 57.3 degrees apart and cut it out roll it around tape it together and check it out. if it looks good make it out of steel or whatever you are using.