In moderate rain or when there is a leaf (or more) on the grate, most of the water will splash through the hole. I would rate 50% or more. Installing them is to prevent leaves from entering your drainage system. Leaves get caught between the grate and the hole on the grate due to the 1/4 inch lip, and water hits the leaves and squirts out. This cannot be prevented. Even in light rain and with a grid, it is completely transparent, about 10% of the water splashes out. Good idea if it worked (but it doesn't).
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