There are very few people that I would trust to distinguish Heniochus diphreutes from H. acuminatus (perhaps Jack Randall and Scott Michael). I know I can't tell the difference and I've been staring at them for years. Although I've read references to the length of the middle black bar and the shape of the mouth, according to Randall, the only difference is the dorsal fin ray count. I think these other supposed physical differences are more related to size and not species. So, I think it will be complete luck if you do get H. diphreutes. OTOH, the odds are in your favor because H. diphreutes, which is a schooling planktivore, is more common than H. acuminatus, which generally swims alone or in pairs and feeds on sessile invertebrates.
BTW, I can post a picture of what I think is H. acuminatus for reference if you're interested. This fish was ID'd by Scott.
Good luck!
Greg