It depends on what you mean by high flank if just appears high know but not like some one put a belt on him and cinched it up as tight as it will go. I would do as stated above feed a coarse hay free choice and use a filler product at show, some calves as they reach the fat stage will drop their flank, over the last 2 years we've had 2 steers do just that, worry that their to high flanked and then bang they start finishing out and it all balances out.
If you could post a picture it might help determine if it is high flank or just the stage of growth the calf is in.