Take the 20%.
Given your 'design' was simply the idea of arranging the circles so that the slowest was outside like an orbiting planet, that you mocked up using one of his existing watch designs (which already used rotating discs and circles just in a slightly different fashion) I would say that 20% off is a generous offer from him.
Given your 'design' was simply the idea of arranging the circles so that the slowest was outside like an orbiting planet, that you mocked up using one of his existing watch designs (which already used rotating discs and circles just in a slightly different fashion) I would say that 20% off is a generous offer from him.