You'll get the most out of it if you feed it everyday. IMO pretty much all feeds and supplements should be fed on a daily basis. The horses digestive track is naturally built to constantly be digesting forage, not complete feeds or feedstuffs. That's why it's so important to gradually introduce new feeds to a horses diet - their digestive tracks need to adjust to the different starch, fat, and protein levels that are not typically found in forage. Once the horse has been fully introduced to a new feed, it's best to feed the same meal each day.
Once you've started feeding the Enrich & flax (and free choice hay if possible, you'll see optimal results this way), give it few weeks to see if you notice any coverage starting over the ribs. If my assumptions are correct then this should be a sufficient amount of feed, but if he's a harder keeper than I suspected you could add a few cups of beet pulp or alfalfa pellets to the mix, you'll just have to play it by ear.
I also converted the weights I suggested into cups for you:
2 lbs (4 cups) of Enrich 32
1/4 lb (3/4 cup) of whole flax seed
I apologize for such long, ranty posts. My horses have been on the same feeds for so long I think I was deprived of playing with different ideas...