I am still very new to this too.... Out of all the ones I have refilled so far... I over filled 3 and it was with a thinner type juice. (lol.... It was also very late at night when I did those... so who really knows how many I put in) Now I know with that juice, even though it absorbs quicker... not to use as many. As for the others, the juice was a bit thicker and I didn't use as many drops. I am still guessing at the amount of drops for each. Usually I use between 15 and 18 drops... It also depends on how dry the filler is.
Also, I always let mine sit (usually overnight). Yes you should always be able to hold it up to a light and be able to see light all the way down the middle.