This is how my tiny little brain has thought this one out.
A is the length of line that drops the weight
B is the length of leader from separation of lines to the hook.
If you have a swivel separating the lines, then these are fixed lengths. If you have a slider, then length A can change depending where you put the slider stops.
A+B = length of line between hook and weight, has to be greater than 18"
if length of A is less than the length of B then the weight is ABOVE the hook
if the length of A is greater than the length of B then the weight is BELOW the hook