you can always use this simple formula; C1XV1=C2XV2 where C1 is original stock solution (10mg/ml), V1 is volume you take from original stock solution (25ul), C2 is the new concentration you are looking for, V2 is the new volume (975ul+25ul). Hence, 10x25=C2x(975+25); C2=250/1000=0.25mg/ml. Amina Mohamed Loulah
There are many online calculators to aide you with this, above replies show the basic math behind these calculators. Just google "dilution calculators" i.e. https://www.tocris.com/resources/dilution-calculator