There's nothing to measure, it's subjective.
Legally, a “box junction” means an area of the carriageway where the yellow marking has been placed and which is at a junction between two or more roads.
You would need to successfully argue where you stopped is outside the junction with the road on the right, and also that there is no longer a junction with the road on the left as it is closed.
Some adjudicators allow the council a lot of leeway, others are more sympathetic to you.