In the script section Settings is a variable called SHINYPOKEMONCHANCE. This is the rarity of shiny Pokémon (8 by default). The odds of a Pokémon being shiny will be this number divided by 65536, so the lower this number is, the rarer they are. In the most recent version, the number is the double = 16.
I've played Pokemon Reborn and Rejuvenation for quite a while now and they put the number at 700/65536 which is about 1.07% and it's truly enjoyable people are loving it !