What is the correct flour, sugar, butter ratio for crumble?
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.