How much is too much?

My TB is turned out on grass 24/7, and while it's quite dry (middle of summer in Aus, and it's HOT!!!) there is still a lot of it, up to your knees.
I've been feeding him once a day to try and get that bit of extra weight on him, esp before winter sets in.
BUT... how much is too much per feed? I know the ideal is lots of small meals and that over-feeding is as bad as not feeding enough, however as I don't have my license yet (only 6 more months!) I rely on my parents to drive me to the paddock, and they'd be none too keen on taking my up there twice a day, once a day is hard enough.
Anyways, I've been giving him 2 scoops (2kgs) of this stuff
Mitavite - Scientifically Balanced Feeds
With a bit over 1 scoop of mixed oaten and lucern chaff, plus various extras, but they don't add to the bulk so they don't matter in regards to the question.
Last night he didn't finish his feed, so I wondered if he was getting too much? Should I lower the amount of Economix and up the amount of chaff? Lower his food intake altogether? I don't know. I can post pictures if it would help.
Thanks

Their stomach can only hold about 5lbs(2.5kg) per meal ... so you want to keep the feeding amount under that