My dear Alhassan,

it is simplest %-caclulation:

your % yield is calculated via the ratio of extracted mass (here 0,5mg) divided by the primary starting mass (here 250g) times 100:

%yield = 100*0,5mg/250g = 100 * 0,5*10-3*g/(250*g) = 2*10-4

Thank you so much for the insight given me in the calculation.