Duwe6 should check his calcs. 3/16" Plate weighs 7.65 PSF which equates to 0.85 oz./sq.in.
Using a "heel" of liquid does not comply with API 650. You must satisfy Par. 5.11.2, both criteria.
The moment of liquid (MF)in formula 2, uses an annulus around the inside, 1/2 the height of the shell...