My Simplex Noise node is nearly finished, but I'd like to add an output that generates a bump vector from multi-sampling so I can include effects from the Function input, along with any crazy things the user inputs in the alpha channels. (The regular, faster, bump vector output uses the direct-gradient calculation made by the simplex noise generator, but that can't incorporate the Function input and nor possibly the contrast setting.)

According to this article (section 5.6) I can do this using only three extra samples by getting the values from three surrounding points from the main spot coordinate by adding a small epsilon value.

What I can't figure out is what value to use. Is there some commonly-accepted constant I should use? Do I need to generate it somehow?