Dark spots on fan leaves, what does it mean please

Half strength is good, and judging by the dark green colour of the leaf in the photo, she is certainly getting more than enough nitrogen. Spots like that are generally an indication of a cal/mag deficiency starting, 2 elements that cannabis uses quite heavily during flowering. There is no "quick fix", as the problem occurred gradually, so must be the remedy. I say this because some people will give double strength or double the frequency of application to try and make the problem "go away", which does not work. Follow the instructions exactly and in 3 weeks or so the problem will disappear and in the meantime, the leaves will still look funny, but they won't get worse. Hope this helps, Organoman.

Looks like a phosphorus deficiency.

Your leaves at this stage will start supplying the plant with certain elements that she may want a little more of. It's normal to start losing leaves 5 weeks into bloom.