There are many great websites out there, Pokémon Database included, that show you the rarity of the Pokémon found in the route, and even in certain parts out the route. Other great sites are Serebii and Bulbapedia. Most of this is correct. To extract this, simply type in the route, then it will show you items, trainers, and the Pokémon found in the route. It will most likely be separated into grass, water, fishing rods, and talk grass, depending on the route. Next to each Pokémon will be a percentage, which represents the probability that you will encounter that Pokémon in any wild battle in that terrain. For example, if a Magicarp has a 90% next to it under old rod, you have a 90% chance (0.9, 9/10) of finding that Pokémon in any wild encounter. In a perfect world, all the percentages will add to 100%, so you will plug in the percents for x for each Pokémon in each terrain type.