It's not at all that simple RUFUS. It all depends on the humidity in your house rather than the external environment. A modern house could be as low as 20% whereas an older property, like ours was in the UK, it could be over 80%. We incubated dry both in the UK and in France because of the high humidity in old brick and stone properties. However, our aim was to get the air sac development right, which is the crucial aspect- in the last property in France we added water for a few days because the air sac looked too big. Must add that the ambient humidity in that property was 70%. Now you are referring here to incubators without humidity control- those with automatic water delivery will do the maths for themselves and just not use any water if it isn't necessary.
There is another factor and that's the natural porosity of the eggs, so some eggs will lose water and develop a bigger air sac than others at the same humidity, which is why you shouldn't mix breeds in one hatch (although many do). The published humidity figures are a 'general guidance' only based on best hatch rates on average eggs- probably derived commercially and not from rare pure breed incubation.
75% for lockdown works for us, but that is subject to hatching times within the incubator and if too many hatch together the humidity rises uncontrollably and you lose some- 'drowning' I have heard it called.