What is the correct flour, sugar, butter ratio for crumble? 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?
There is no single ideal. More sugar and butter will mean a more crunchy crumble top (and one that browns or burns more quickly), more flour will make it more sandy - they each have their place. The juicier the fruit, the sandier I like the topping to absorb some of the juice. The longer the fruit needs to cook, the more resilient to cooking the topping needs to be. And of course for a tart fruit like rhubarb, you might like more sugar in the topping to balance it - or you might just add sugar to the fruit and leave the topping alone.

My goto mix is 1 cup flour, 1 cup sugar, 1/2 cup butter. But play with it a bit and vary it according to the fruit you are using - and I don't just mean "rhubarb" or "apples" but taste a bit of today's fruit to see how sweet it is, how soft it is, etc.