Hello Richard,
To get your head around this issue its worth bearing in mind that you are getting yourself in a twist about a debatable 15 thousands of one inch! i.e. your quoted 0.4mm difference between front and rear treads. A !Removed! paper as we used to say.
Now in theory you can run the fronts down to an even 1.6 mm remaining, but come what may, you will need to bite the bullet and renew the set since you will otherwise be at the worse possible case of maximum tread on the front and minimum tread on the rears until you change the rears some weeks later.
Kind regards,
Gareth.
Whats wrong with the three letter word "!Removed!" then?