I'm not able to find numbers anywhere that indicate what air flow rate is comfortable vs a bit drafty/noticeable.

The situation is that the ground floor has no corridor and one huge lounge that you walk through to reach the other downstairs rooms - so all the air going into the 3 bedrooms upstairs is going to need to go through the lounge in order to be extracted via the kitchen and downstairs loo extract valves. Is the guidance somewhere suggesting a particular limit for flow rate?

Design is getting finalised soon but the lounge has 44m3/h coming through a single door undercut from upstairs, plus 34m3/h from the supply valve (meaning 78m3/h leaving the lounge via a single door). That sounds like a lot to me. The lounge is about 20sqm with fairly low ceilings. I'm super sensitive to air movement /draughts even if they are warm air, so is this flow rate too high? We could reduce flow through the lounge by adding an extract valve in the entrance hall/stairs area, before it reaches the lounge. Or we could increase the flow to the bathroom extract.

I think to get a meaningful response you'd need to add a few more pieces of information
- ceiling heights. You say these are low. What are we talking 2.3m, or even lower?
- occupancy. How many people are likely to be in the living space?
- flow rate of supply extract terminals involved. I got a bit confused where the air was going to and from in your description above.

The passivhaus guidance is to aim for 0.3-0.4 air changes per hour, which I think would be a bit lower than the flow rate you describe above, but is also guidance for the whole building rather than a single room.

Most of the industry guidance is based around relative humidity and how 'dry' the air feels rather than physical draughts. In my experience, at Passivhaus flow rates you only really feel the air flow if you are holding a hand close to the terminal. There is something called the Coanda effect that means (I think) that air supplied at high level near the ceiling will stay buoyant due to static pressure in the room and be thrown across the room rather than falling immediately and causing draughts.

There's more on this here if you want to get into the deeper physics & maths:

https://www.cibsejournal.com/cpd/modules/2022-12-air/