Drops don't make a reliable unit of measurement.
Depends on the VG itself. There is almost waterfree VG around and there's brands with like 14% water conent. Small print on the bottle should tell you.
The latter is usually good for
vaping as is.
A ball park figure to start with would be adding 1.0 ml to that bottle for starters and see how it wicks. That in turn depends largely on the setup you're using.
Cuz too thin a liquid is no fun either - offten causing excessive leaking. If it's still too gooey, add a little more ( fractions of ml )
If that 15ml is flavoured already do bear in mind that adding water will also dilute the flavour...