I personally have never added THH to changa so I can´t say from experience, but my recommendation is to make two small batches, one with half the amount of THH than the other. So for example lets say say you do for spice:
plant:harmine:thh two batches, one with 1g:1g:300mg:300mg, and another with 1g:1g:300mg:150mg . Then you can try both, and see what you like the best. If for example one is way too diluted and doesnt have enough THH, or maybe the other is way too concentrated, you can always dilute or increase concentration afterwards to what you desire
Also I´d ask you to please share your experience here so others can know what you thought about the different concentrations
Good luck!