I may be mistaken, but I believe I remember suggesting approx. one half cup. It will not raise the pH overnight (which is good) but will take several days to begin affecting the alkalinity . You can test the water (pH) every two or three days and take note as to the rising pH. If after ten days to two weeks, the affects are still not raising the pH to something more suitable for live bearers 7.4 to 8.0 then you can add a little more. Best to place the crushed coral in nylon mesh bag or pantyhose to keep it from getting loose in the aquarium. You will need to clean the crushed coral once a month maybe longer ,when it becomes covered with silt,bacteria,and or algae or replace it. Once it becomes covered with these substances it is far less effective.
I can't remember how large your tank is, it may be possible to start with one measured cup of the coral in larger tank. As mentioned the effects will be gradual which is good.,,
otherwise sudden change in pH or alkalinity would have bad effect on fish. Give the coral plenty of time to do it's thing before adding more if needed. Always continue with weekly water changes but changes should not be too large.