The useable width is determined by taking the width of the formation at its widest point (in this echelon right, 4 A/C and adding the width of a TDP; 50m in this case. The useable width of your site is 156m. For any formation that is as wide as it is long, you will be able to take the useable length (before adding anything for a NATO T if that is required) and add a TDP size to it.