Peter, I understand your concern, and I share it. It's as bad a "juice of half a (lime, lemon, whatever)". But what can be done? It's easy to specify a volume of juice, but what could be done about leaves? And what about the variability of muddling technique? I don't see a solution. For me, I sometimes make dated notes on the recipe, just as I do when 'tuning' someone else's recipe to my (or my wife's!) tastes. Sometimes the outcome is multiple variations. In my case, that would include margaritas and daiquiris.

¼ barspoon of xanthan gum?
If you are lucky enough, like me, to own an Easy Jigger, and the author was referring to an 'American barspoon,' then you can measure 1/24 ounce.

If a Barspoon = 5 ml
then ¼ Barspoon = 1.25 ml = 0.042 oz (approx 1/24 oz)

If a Barspoon = 2.5 ml
then ¼ Barspoon = 0.625 ml = 0.0211 oz (approx 1/48 oz)

If a Barspoon = 2.4 ml = 1/8 oz
then ¼ Barspoon = 0.6 ml = 0.02 oz (approx 1/50 oz)

Bourbon, Campari, Aperol, and lemon juice make a fine Paper Plane! Some folks on the west coast are replacing the lemon with lime, calling it Ruby Sippers.

I plan to look for Luxardo every time I go into a bar, and when I find it, I'll taste it. I saw it in so many recipes that I bought a bottle. I think it's nasty. My wife thinks it's disgusting. So I keep wondering if I might possibly have gotten a bad bottle. But I'm not going to invest in a second one to find out!