My mother is an OB/GYN, that's where I get the 1 in 400 rate from. Quick googling will tell you that the identical twin rate is 3 per thousand ( https://www.verywellfamily.com/identical-twins-2447126 ) or 4 per thousand ( https://twinsmagazine.com/incidence-of-twins-by-twin-type/ ), or half the rate of fraternal twins (the first link again, but this is false -- the rate of fraternal twinning depends on the parents' race, but the rate of identical twinning doesn't).