The rules are pretty much the same as previous games except odds of things happening.
Change 1 - Encountering a Pokemon on the edge of a patch of grass does not change your odds of continuing a chain.
Change 2 - Shiny charm does not impact PokeRadar, and the odds of getting a shiny with a chain of 40 is 1/99, not the prior 1/200.
Change 3 - The odds of continuing your chain after defeating a Pokemon that is 4+ squares away is 83%, and if you catch the Pokemon it is 93%. This means the chance of getting to a chain of 40 with defeating Pokemon is 0.05%, and if you catch 40 Pokemon it is 5.49%.
Other than that I would recommend watching this video, as it goes into every detail of PokeRadar hunting in BDSP.