The flour:sugar:butter ratio I have settled on is 1:0.7:0.7
Out of 116 crumble recipes reviewed, I found 58 unique flour, sugar, butter combinations. At the sweetness extremes, there is a recipe with only 6% the weight of sugar to flour, and another with a 1:2.4 ratio. Regarding butter, the ratio range in my sample goes from a lean 1:0.1 through to 1:1.9
Despite an apparent lack of harmony the ratios show, on average, a very clear tendency towards 1:0.7:0.7
I baked a (blueberry) crumble using this ratio and...well actually I burnt the crumble enough to invalidate the experiment. It was still good eating, but I'll need test this ratio again when I'm ready for more crumble. Comment feedback is appreciated from anyone who tries this ratio themselves.
Update: Kate Gregory's excellent answer prompts me to suggest a range of workable ratios based on deviation from the mean:
- Flour to sugar range: 4:1 to 4:4.6
- Flour to butter range: 1:0.4 to 1:1
These figures are not backed up with so much as a single experiment and about 37% of the 116 recipes in my sample are outside of these ranges. So take them with a pinch of salt (both literally and figuratively).