The 6.75" requirement is just the distance between the top and bottom
mounting, as it were, it doesn't all have to be wood.

Ahh... okay, that makes sense

It is much easier to fit than noggings, as any slight error in length is
accommodated by adjusting the angle of fitment.

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...