Search for "metered pour spouts" or "measured pour spouts". They are type of speed pour bottle nozzle that uses a special check valve to meter out a specific amount of liquor in each pour. You could get a 1oz metered pour and knowing the bottle capacity pour out only the amount you designate while leaving a known amount behind. This should work well for your infinity bottle in both drinking it and adding to it. You can get them in lots of portion volumes and they are priced very reasonably. You may want to invest in some all stainless steel ones to keep your whiskey untainted though.