My interpretation of the other question about the ideal flour/sugar/butter ratio for crumble, is that the flour:sugar ratio should be somewhere between 1:0.7 and 1:1
Now, if you add oats to the crumble, how much should you use? Currently I just add oats by eye, about 2-3 heaping tablespoons to a cup of flour. Quickly measuring this out, I'm getting about a 1:0.25 flour:oats ratio. Is that a reasonable amount, or is there a better ratio?
To directly answer your question: if you like the results with your ratio, it's reasonable. If you like something else better, there's a better ratio. And making the topping by eye is definitely the right way to go; I've essentially never used a recipe.
Most recipes will have at most as much oats as flour (i.e. half and half) but any ratio is fine. It's entirely personal preference. You can make a topping with all oats and no flour, or all flour and no oats, as long as you add enough butter to make it hold together properly, and enough sugar to make it as sweet as you want it. It's a little harder to get oats alone to stick, so you can use a tiny bit of flour if you have trouble.
As an aside: you might find that you also use more flour if you add nuts, since your preference might be for a certain combination of cookie texture with things mixed in.
Bittman lists 0.5C flour to 0.5C oats to 0.33C butter to 0.67C sugar and about 1.5 quarts fruit. (C = cups).
Clearly you can vary these quantities and ratios.
