I don't know the percentages and am not sure if anyone has compiled that data.
Using my dogs and their lines as an example though, I think that there are many factors that influence the range of size we see. My dogs all go back to Kenset dogs for at least half of their pedigree. Nicky, my male, is about 17 1/8 inches tall at the withers. Rally is just shy of 16 1/2 inches, Rio is 16 1/2 inches, and Sophie is just shy of 16 3/4 inches. The male siblings of Rio and Sophie are all somewhere in the 17 1/2 to 17 3/4 range last I saw them.
Now this year, I leased a girl, whose pedigree is also heavily Kenset. TC is closer to 15 1/2 inches tall. So are most of the girls in her breeder's house, and all of them are at least 50% Kenset in pedigree. Most of the boys she has run tall, close to 18 inches.
So even in the same lines you can get differences that I think are influenced by breeder selection. Tomorrow we will find out if TC is pregnant, if she is, I would expect that the pups will all grow to be within that +/- 1 inch of the standard but it will be interesting to see how their heights turn out as adults.