It is not uncommon to see Bijou recipes with equal parts gin, Chartreuse and sweet vermouth, as called for in Harry Johnson's Bartender's Manual. In fact...
I hope the shortbread garnish was just in the general vicinity of the cocktail and not dissolving in the bottom of the glass. Agree with T. Cull that 30 ml is just as easy to measure out as 25 so went with a full ounce of gin (Hendrick's). A full ounce of Chartreuse would overwhelm the drink, however, while half an ounce provides herbal complexity but probably better balance. And while the Jamie Gordon Bijou seems in retrospect a bit like a Negroni-with-added-Chartreuse, I went with Dopo Teatro to get some bitterness along with the sweet ingredients; this was incredibly good, 5 stars. Happy New Year!