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! 
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!