I thought 20 drops was 1 ml.
That's a "standard" measure. I have a couple different pipette sizes, and I've counted about 30-35 drops/ml for one of them. The key is CONSISTENCY. Once you know YOUR drop size, just stay in proportion.
The DIY calculators allow you to modify your number of drops/ml to help you stay consistent. (I'm just too stinkin' lazy to count