Some false information in here. But mostly correct.
First and foremost. The single best way to find out the coefficient is just go to wowhead and look up the spell and rank in question.
https:/ /imgur.com/a/K5tuEXJ
As seen in the picture there is an (SP mod) that is your coefficient. Dots use a lower % coefficient as its a per tick coefficient vs normal spells which is a 1 time coefficient. If you look up a spell and it has 0 coefficients that means its a 100% coefficient you will get 1 point of damage per sp.
This is false. It depends on the rank of the dot and what dot it is. For example the above SW:P I linked has a total coefficient of 0.402. Meaning over the whole cast of SWP it only gets 40% of 1 Spell power. But later on at max rank for example it does get 100% of the SP.
Then you look at moonfire and even at max rank of 10 it only gets 52% coefficient (67% if you add the initial damage)
Its not a level thing. Its a spell rank thing. If you use a rank 1 frostbolt at level 60 its still going to have the same bad coefficient.