Any given dead leg should be a max of either 2 x the pipe diameter according to Water Regulations Advisory Scheme or 6 x the pipe diameter according to The Chartered Institute of Plumbing and Heating Engineering. That's two professional bodies giving out different info. Nuts. https://www.wras.co.uk/consumers/resources/interpretations_and_advice/installation/i21/ CIPHE dead leg guidelines state that the length of any dead leg should be limited to six times the pipe’s diameter, but even shorter dead legs are preferred. This is known as the six diameter rule. However, in practice, it is both safer and more practical for dead legs to be limited to three pipe diameters in length, as five or six pipe diameters could present cleaning difficulties.