there is no percentage, try repel trick on a rare poke to understand that. You will find 100/100 rare poke but you will realise that it's still not ez to find...
each spawn is more like an average number of steps to do before to find the poke and they are categorized in 9 tiers, but a tier 9 can be 100% spawn or 0.1%
here you can find tons of data about spawn rate
https://pokemonrevolution.net/forum/topic/100579-url