I do know one thing thanks to the law of Mr Sod, if you do not
allow a
bit of extra on a run, something will come along and mean the
whole run
will need to be done again when the thing is just a little too
short

What to do with the excess length?

You "lose" it along the run, any changes in direction are lazy and go
wider than they need to but still remain neat. You have a generous 6"
of striped cable(*) in the box behind any accessory, you have used 35
mm deep boxes haven't you?

(*) Well for sizes up to 4 mm^2.

--
Cheers
Dave.