Let's assume your jigger is dirty. I can make this assumption because my jigger is always dirty. Not just dried-tequila dirty, I mean I'll-put-it-in-the-sink-so-it-doesn't-drip-all-over dirty. Which means a-week-worth-of-dishes-piled-on-top dirty. Does this mean I cannot have a properly-measured cocktail after a hard day at work? Bah! You know me better than that.
Oh, food scale. Your usefulness continues to amaze me.
Place your old fashioned glass on the scale, press the "zero" button, and commence pouring your homemade moonshine rhubarb-infused gin until you reach the 1 1/2 ounce mark.