I started with the undersized sprue that measured 0.200 inch where it attached to the two gates feeding into the part and 0.180 inch at the sprue O-diameter. I measured the length of the sprue and it turned out to be 3.25 inches long. Why was this significant? Well, I like to use at least a 1½° taper on the sprue, and for a sprue that is 3.25 inches long, I should see a taper of 0.057 inch (not 0.020 inch, as I measured).