How do I work out the size of Shadowline skirting to use?

The sizes of the Shadowline skirting are now shown as the visible height, NOT the total skirting board height. This allows you to simply measure from the floor up to the top of where you want the Shadowline groove to be positioned. That will be the visible height (as the top of the skirting is hidden behind the plasterboard). This diagram and Shadowline table of dimensions provides you with all the dimension options to help you select the size that best suits your requirements.