Using my fast coffee scale for cocktail creation is efficient and eliminates the need to wash my jigger. I round to 30ml for one jigger and 15 for a half. This is easy to remember as those are the volumes of a standard espresso shot and a ristretto respectively.
Cheers! 
At the risk of being pedantic it is important to note; you are not measuring volume with a scale, you are measuring weight. The ml to g equivalent is only in cases where you are measuring pure water (at 4°C). Scales that have a ml setting, are assuming you are weighing water, unless indicated otherwise. So your volumes will be off depending on how light or heavy whatever you are weighing is relative to water. Alcohol is lighter than pure water, therefore you will end up with more ml of it, to equal what water will weigh. However at such small amounts this difference may be negligible. But with harder liquors small differences might be worth considering.
Correct, you need to calibrate with the desired liquid. One jigger of 80 proof spirit is about 30g for me. There is quite a bit of variance across jiggers though. Also this method is more accurate as you never really fill the jigger completely or you’ll make a mess and the degree of unfilled volume varies a bit pour to pour. When in doubt with spirits, always round up!
Þe best kind of pedantry is measurement.