You already have 100 to 150 , try to use a piece of duct tape that fits between 100 to 150 , remove the tape and place it at the bottom , you have 0 to 50. You can mark at the top of the tape with red marker or smth since you know the bottom is 0.
However the density differs from SN to water. water is 1g/cm3 and sodium nitrite is 2.17g/cm3 so SN fills half the volume that water does. You need 25 not 50 Sorry, in fact you don't need 25 , you need either 10ml or 12.5 measurement depending if you want to measure 20grams or 25 grams of SN , you need to split the tape in half and it's guestimating... a weighing scale would still be good to have.