You may be surprised to see absinthe in this recipe. I was talking to Ryan Maybee of Manifesto in Kansas City. He told me when he was at Harry's Bar in Paris he was drinking a French 75 and noticed a hint of anise in it. He asked if the bartender had put absinthe in the French 75 and the reply was a yes.
That is how Harry's Bar makes their French 75, and they are as close as you can get to where the drink originated. So I have adopted this addition for my recipe. I think absinthe can add entirely different layers of flavor to a drink in small amounts and that is the case here.
Hemingway saw fit to add absinthe to Champagne directly (Death in the Afternoon). So why not in a Champagne topped gin sour?
You may increase the gin to 2 oz if you prefer. But if you are using English gin (which you should) at 94 proof and you're topping with Champagne, 1.5 oz of gin should be sufficient.
Also, you can make a French 75 in a Champagne tulip glass without the ice. This version is nice if you are inside at night only dealing with a fireplace that is trying to warm your drink. If you are outside in the sun or just want a cold drink do the iced version in the highball glass.