Eggs should really stay nice and plum throughout most of the incubation process, mine always start dimpling during the last week of incubation sometimes (rarely) starting 2 weeks before hatching.
And don't try this at home but I actually do not measure humidity anymore, I simply have it down to where I know that the humidity is high enough. I have been using the same incubation method for several years tweaking it around to the point that now I have perfect humidity (not too low and not too high which can create too much condensation which can drip back down on the eggs)