If they are 400mm centres, just start your joists at one end of the garage ceiling, with your first joist close to the wall.
Then do your 400mm centres for every joist, taking in to considerating your first measure will be from the wall and not the centre of the first joist.
Then when you come to your final joist ,if the distance is smaller than 400mm so be it.
Your joist should be every 400mm
Extra studs?
For what?
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