I've not made any bed size quilts, so I am speaking hypothetically. Rounding off the numbers, you should have about a 20" drop at the bottom and about an 18" drop on each side. Of course, batting shrinkage will take some after it's washed. Someone above says you'll get some shrinkage just from the quilting.
I found this, which I will bookmark (It seems pretty comprehensive):
https://www.generations-quilt-patterns.com/support-files/quilt-sizes-worksheet.pdf
bkay
edit: Maybe you made the same mistake I did when I started calculating. I didn't stop to think there is only one drop on the length and two drops on the width.