But the length, andgle and distance between the joists are all related, so
if I cut slightly too long, the angle will no longer be correct for the
400mm...spacing. Fitting it at an arbitrary angle will mean the strut wont
butt nicely onto the joist. It probably doesnt matter, just seems like
shoddyness...

Obviously you should try your best to be accurate. However, 2mm won't make
the slightest difference with herringbone. With a nogging, 2mm inaccuracy is
as wide as an ocean.

Christian.