Eye droppers are cheap, and should be part of your tank gear. 20 drops of Prime equals one ml, for larger amounts the kid's medicine droppers can't be beat, they're graduated in ml. They're the tiny version of a turkey baster, another piece of tank gear that you should have.
I remember those little droppers Seachem put in the bottles, I've got one in one of the cabinets. You know times are tough when they have to cut out a little accessory that maybe cost them a nickel.