I have a dendrophyllia, which is pretty similar to a sun coral. I have had mine for almost a year now...went from 1 polyp with 2 babies to 1 polyp with 3 medium polyp babies. I have mine in higher light just b/c it's easier for me to feed. If you have algae issues, I'd put it in a really low light area so algae doesn't over take it. Flow is medium...high enough to get food to it when I'm not feeding directly...but low enough that the food will stay in its mouth. I also have the coral where crabs/snails/etc have a hard time reaching...so they won't steal the coral's food.
If you have some trouble getting the coral to extend its polyps, try putting in a drop or two of amino acids about an hour before feeding time. That'll get the coral to open up wide.