I soak all pellets, I never feed anything dry.
I got that from a very, very canny lady, whose show horses were always in such fantastic condition, young, old, stallion, mare , gelding, ALL shone like the sun.
I asked her how she did it and she told me "Turnout on good grass, every single day, whatever the weather, even on a show day, for at least two hours minimum, and soak all the feed" She only fed stud pellets, and she soaked them and mixed them with chaff (as I do) and fed them as a "long" feed.
She had Anglo Arabs, and at the time had the premier stallion in the country, so I took her advice!
It has never failed me.
I would be inclined to feed her as much (within limits!) as she will eat, and you could easily go up to one full cup.
Does she have grain with this, as well?
And how much hay is she actually eating, is she picky with that, too?