I do the same. At first I measured the right amount via a 3cc syringe, and then i inserted that into the dropper to fill it with the correct amount. I then put that back into the clen bottle and drew a line with a sharpie on the Clen label to indicate where I should fill the dropper up to.
Makes measure fast and easy. Of course theres a good margin for error, but its close enough for me.