Try - BS EN 15154 I would say - is distance and accessibility not more important than ratio - surely only one person is going to need to use them at once? Not being able to reference your risk assessment makes this tricky, As I would consider within that doc... -number of time showers used previously -types of incidents that would require shower -substance involved - speed it will damage skin - difficulty to wash from skin (does it pass straight through skin etc etc) -how many people likely to be affected (i.e. is more than one shower required at each location) -max travel time to shower (consider op who can't see!)