I'm putting a radiator in my conservatory so I can use it on the odd
day in the winter so I'm going to add Fernox alphi11 antifreeze to the
system. How much do I need to add? They reommend a 25% mix, is there a
quick way to calculate the ammount of water in the system so I don't
have to drain the whole thing down.

Before anyone asks part of the system has stop valves so I don't have
to drain down to fit the new rad.
Thanks