You will be calculating a volume that is described as a "frustrum of a cone" where V = volume, in your instance removed, R is large diameter of the cone, r is the small diameter of the cone, h is the height or depth of the cone and of course pi = 3.14. 

V=(pi x h)/3 x(R² +Rr +r²), this the volume removed then you need the original volume = radius²pi x height (or depth) , subtract the frustrum of the cone from the cylinder volume and you have the volume left in the bin in cubic feet then divide by 1.25 for the number of bushel.