122-(x*11)
where x = number of steps

ROUTE 1
0-50 = L3 Pidgey (assign 25)
51-140 = L3 Rattata (assign 95)
141-165 = L2 Rattata (assign 153)
166-190 = L2 Pidgey (assign 178)
191-228 = L3 Pidgey (assign 209)
229-241 = L4 Rattata (assign 235)
242-252 = L4 Pidgey (assign 247)
253-255 = L5 Pidgey‏ (assign 254)

ROUTE 22
0-50 = L3 Rattata (assign 25)
51-101 = L3 Nidoran-M (assign 76)
102-140 = L4 Rattata (assign 121)
141-165 = L4 Nidoran-M (assign 153)
166-190 = L2 Rattata (assign 178)
191-215 = L2 Nidoran-M (assign 203)
216-228 = L3 Spearow (assign 222)
229-241 = L5 Spearow (assign 235)
242-252 = L3 Nidoran-F (assign 247)
253-255 = L4 Nidoran-F (assign 254)

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Remember -384 in overworld / +768 in battle