Looking through a number of fruit crumble recipes I see that there is a wide range of ratios used for crumble topping. Some use equal amounts of flour (or other dry ingredient), sugar and butter. Others use about half the weight of butter and sugar to flour. Others still use double the amount of sugar and butter to flour.

Is there a definitive ingredient ratio for crumble as there is for pastry (3:2:1)? If so, what is it?