Useful levels with Rare Candy strat (Gold)

Assuming no spinners:
Lv. 40 for Porygon
Lv. 40 for all 5 Koffings + Weezing
Lv. 41 for Rival 4's Golbat
Lv. 41 for Rival 4's Meganium
Lv. 42 for both Muks
Lv. 44 for Golbat
Lv. 44 for Arbok
Lv. 45 for Houndoom
Lv. 48 for Kingler
Lv. 54 for Dragonite
Lv. 64 after Charizard (+3 seconds)