Just To Make Things Difficult

A large lorry carries an eight foot wide tailboard at the rear.

One day it is being manoeuvred in a small square yard when the driver makes a mistake and backs it into one of the corners at some angle.

What is the distance between a point at the centre of the tailboard and the point at the apex of the corner? (assume the corner is formed of two flat brick walls meeting at 90 degrees and the tailboard just touches both walls.