My thought: Weigh the egg and reduce the water amount by that weight. I think there's some info on DeeAnna's website for egg soaps that might be a good reference if one wanted to dial in exact water content.
Wait, the recipe spells out the amounts of the water and the lye that they want you to use.
It doesn't make sense that they would give you specifics, then tell you to do math to change those specifics. Thus, I believe the reference to "6% discount" is probably an informational reference to the final superfat number (aka lye discount).