Why are you going with crushed coral? Just curious
If you're not using the sand bed for denitrifying or to house certain fish or inverts (pistol shrimp, sand sifting goby, etc) and it's purely for ascetic purposes, the choice is really up to you as to how much you want to use. Just know that crushed coral can be harder to keep clean than sand and will limit certain choices for inverts, including corals (I wouldn't want to put a Goniopora or Tracyphyllia in crushed coral substrate)