The dividing line between a cocktail enthusiast and a craft cocktail aficionado is knowledge. Anyone can enjoy a cocktail, and with enough time spent at or behind a bar, attain a good enough working knowledge of brands and flavors of alcohol. Some of these people go on to create a new cocktail, usually starting with a common drink or ratio of spirits to other ingredients and tweaking them to make something pleasant.
Alas, this approach lacks repeatability in creating quality craft cocktails. Craft cocktails are not dump buckets for every neon colored, super sweet liqueur that your distributor is pushing. And they are not made to mask the flavor of alcohol, but to support and sustain it.
Combinations that should work based on the flavors of the components often fail to impress in the glass due to a lack of understanding of those same component's structural elements. Skilled mixologists construct cocktails from some basic building blocks: alcohol, sugar, acidity, and bitterness. A thorough understand of these primary elements can help craft cocktail designers make great drinks with a minimum of waste and trial and error.
The cocktail dates back to the earliest parts of the 19th century. The original contained no acidic agents – just a simple mix of spirit, sugar, water, and bitters. But by 1862, Jerry Thomas had entire sections for Sours, Fixes, and Daisies, all of which featured acidity prominently. Certainly, the use of spirit-plus-acidity dates back to the 18th century Punch, which were spirit, water, lemon and sugar, with some other ingredients added for flavoring.
If you can find it, use Gracias a Dios agave gin as the Old Tom (the mezcal then becomes superfluous).
I've made this with the full amount of syrup and I've been quite happy, but a bit less also works.