"Gen 3 Baby Pokemon"

#PokemonGO: I think this TSR article is very helpful. I have just 2 questions.1) Why is the divisor 312 in the first table and 315 in the second table?2) If we assume that Gen 3 Baby Pokemon is Ultra-Rare, statistically, how many 5 km eggs do I need to get one of them.3) If we assume that Gen 3 Baby Pokemon is Uncommon or Rare, statistically, then u need to hatch on average 65 (521/8) or 130 (521/4) 5 km eggs to get one of them. Since we don't know the distribution I have assumed the same numbers as in the latest article. Can we do a better assumption?Edit: How is the the divisor calculated? In the latest article about egg species and rarity the divisor is 521.Edit 2: Question 1 solved, ex. 8 species are common in the latast article => 8 * 16 + 36 * 8 + 18 * 4 + 15 * 2 + 3 * 1 = 521Edit 3: Replaced question 2 with question 3Edit 4: Solved! You need to hatch 65 or 130 egg of any distance (not only 5km) to get one Gen 3 baby using assumption above, see comments below via /r/TheSilphRoad http://ift.tt/2CyeQXC