Homebrewer question: seltzers

So my wife is a la croix and truly addict. Drinks 4-5 la croix by 9 am. So I bought her the drinkmate carbonation machine and it has been a huge hit for replacing her la croix addiction. She makes a gallon of some sort of flavored water and carbonates it when ready to drink.

I had the idea to also replace the truly addiction by adding vodka to the gallon pitcher and then carbonating it. The taste is there but I'm unsure of how much vodka to add to a gallon of water to achieve the same 5-6% alcohol of a hard seltzer.

Any idea on how much we should be adding per gallon? Google hasn't been much help.