If the recipe calls for less than 10% fat, add it at the beginning with all the other ingredients. It won't interfere with gluten development by coating too much of the flour granules. However, if you need to add more than 10%, add the butter or fat after the dough has come together and the gluten has at least halfway developed. The excessive amount of fat early on will coat the flour granules and prevent gluten from forming in the first place.
You can determine the percentage of the butter by dividing it by the amount of flour called for. For example, if you have 16 ounces of flour and 8 ounces of butter, divide 8 (the amount of butter) by 16 (the amount of flour) to get that the amount of butter is 50%. (the part divided by the whole equals the percent)
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