I take it that 240ml is required because that is the requirement dictated by either the tank size or because you are using inversion agitation. As a Scot you might want to consider a Jobo rotary processor and Jobo tanks which only need 140ml for 35mm. Saves a lot of chems in the long run and gets temp spot-on

Of course it might be a 120 film which does require 240ml even in a Jobo processor which you might have and you can disregard everything above.