Thanks for the replies. Even within the range of graph, I find that using the graph is a bit clumsy and would prefer to use a formula if one exists.
Ole: My water temperature went out of range (26C) on a particularly hot day. I should probably try a cold water bath next time to keep it in range.
Mateo: I don't recall the Ilford site mentioning that the graph is dependent on the type of developer. I will have to check that.