Mike you can only work out the speed of the water, depending on where you are. The speed of the water is faster on the outside of a corner than then inside. The Strong Stream Boards are put out depending on the amount of weir tackle pulled at the weir. On average with most weirs on the Thames the Strong Stream Conditions come out when approx 50% total weir tackle is pulled. You can use the volumetric flow rate found on the webiste to calaculate an average speed. The other consideration are you insuranced for Strong Stream Conditions, also don't want to stand up in coroners court and try and explain what happened,to many close calls( Pics of boat stuck on weir on the Nene).