The glyph states will cause additional 20% of it's damage over time. However, on testing it numerous times...it only does ~10% of the damage. Is the glyph not operating correctly, or is there something else going on?
Probably bugged, I would try posting this in the bug reporting forums however. U might also want to suggest allowing the glyph to roll it's dot, so that a full 20% is always added.