Funny thing about the white coffee cup - that's what I use too! Pretty sure I read it here.
Mike, I know what you mean about not natural looking blue dye, but I think that's the Aqua blue stuff that does that. The Twilight blue in my pond is pretty black or greyish. Refreshing with True blue looks more natural to me.
Above my coffee cup 24" away is a loop knot, for reference.
My scientific formula for adding more Dye is;
Visible coffee cup + wet knot = 1 quart