[Question] At what shortage should I confront my guy?

I've had a pretty reliable source for a while now. My friend is friends with a guy so I just go through him. I went to school with the guy so he's not a stranger but we've never hung out or anything. I ordered 7g for $80 and bought myself a scale just to double check my purchases. I weighed it out and it's about 6.4. It's not too big of a deal but I'd like to make sure he doesn't do it in the future and think it's alright. At what point would you guys suggest bringing it up? I don't want to create a problem over a little amount.