@brotherdust, I love your observations.
1) Wow - I've never worked it out as a percent. It is because they gain so much at the start that niacin and other vitamins simply cannot wait! I had observed that ducklings fed ample niacin during the first couple of weeks seldom show a deficiency later - but when supplements are started after a couple or three weeks, it's almost impossible to catch them up. With the 20% a day figure, it makes sense!
2) I'm going to try your comparison of haunch to toe v.s length. I wonder if this varies between breeds. I also plan to try something like haunch to breastbone vs. breastbone to tail - if I can manage, to see if this is different for runners vs another breed. I'm trying to figure out the exact difference that makes runners stand taller.
3) I have also observed what you have about the two day/no growth is a bad sign, especially if it is seemingly without cause.
I had a duckling that was stepped on. She did not eat or grow for two or three days due to stiffness and being uncomfortable. She grew just fine afterwards and caught up - because there was an obvious cause for her slow growth at that time.
On the other hand, I did once have a newborn duckling that didn't grow at all for ten days and she required tube feeding. After ten days she perked up and grew normally and lived normally and happily for three years. She then died. Autopsy revealed extensive liver/heart disease.