The formula should be written:
(finished size of HST unit + 7/8 ) multiplied by 2
Example: finished size to be 3 inches
(3 + 7/8) * 2 = 7-3/4
Calculators have an "order of operations" - so without the parantheses, this is what you would get:
3 + 7/8 * 2 = 5-1/4