Yeah - if low-carbing, it's all about portion size. Which is the catch when it comes to more-ish popcorn. (As in - one wants more!)
I deal with this by buying pre-portion-sized bags of already popped and salty-buttered popcorn. The bags I buy have six 12g wee bags inside. Each 12g wee bag is nearly 8g of carbs - which is an indicator of how carby popcorn is! 64.5g of carbs er 100g of popped corn.
So, part A of portion control - a premeasured bag. Part B - how often one has nearly 8g of carbs in one fairly modest wee bowl! I allow myself it twice a week, as a netflix-watching on the couch treat.
But! I am not a member of the 5% club! As in, I don't have well regulated non-diabetic range blood glucose. So. Bear that in mind, as I share my popcorn treat dosage.