Yep, drops are a great way to roughly guesstimate. If you want to take the rough out of it, you meter the dropping
device and subject liquid into a graduated cylinder.
I have some 3ml plastic pipettes that average 20-30 drops per ml depending on the flavoring with a median of 21.
Flavour Art bottles average between 30-40 drops per ml depending on flavoring with a median of 36.
Starting out, if you have no better measuring equipment, pick a number and assume you are correct. Since you work with nothing but drops, as long as you are not dropping PG nicotine solution and cutting with VG, then things should be a close guesstimate of the calculated ratios. (Note, PG and VG drop sizes are greatly different and this has led to some unvapable
juice by others during my time here.)