If you have a pre-existing frame (or location) at 3000mm (w) x 2000mm (h) and need to calculate the the size of your banner to allow for the stretch, you have come to the right place.
Using our example size above, you can work out the stretch using the formula;
Banner size minus 40mm for the keder minus .25% (make sure you calculate for both the width and height
For your 3000mm length it would be 3000 - 40 -.25% = 2952.6 (just round it up or down)
For your 2000mm length it would be 2000 - 40 -.25% = 1955.1 (just round it up or down)