My adex dropper doesn't have any markings. I'm supposed to take .25 ml eod, and have been eye balling a quarter dropper, assuming that would roughly be .25 ml. But the other day I tried to fill the dropper to the top and it will only fill three quarters of the way up. Any ideas how to know how much I'm getting? Do most droppers not fill all the way up and is there a general standard for measurement with these research liquid droppers ?