I've never found stress to affect my supply really, more lack of water drunk that day!

I'm no expert though!! This is only my personal thought but I've had multiple let-downs during a feed on both sides, so I would think tugging would be about getting another let down happening. The formula may well feed bubs up but that would then have a knock on effect on your supply, because in the early stages it's critical to have access to the breast whenever needed. Like I said, I fed 2 hourly for months and months, whether it was offered by me or asked for by him.

Your breastmilk is perfectly designed for this stage, it's really dificult to not be concerned about length of feeding and duration with your first (I know!) but you'd do yourself a big favour if you trusted your body to produce what baby needed when it needed it. I found blind faith helped a lot! Obviously eating healthily as much as you can and drinking plenty of water.