when they first start to feel dry on the outside, there is still time remaining.
Moisture on surface and closer to surfact evaporates faster. There is some lag for the moisture deeper to be drawn out (it tries to even out as surface evaporates). Once near surface, it too can evaporate.
So, at this point you have 12-24 hours remaining, or you can go through what is called a "sweating" process.. you put it in some jars, wait for the moisture to equialize in the plant material, then remove from the sealed up container to dry further. Don't let it sit while still too moist in sealed container for too long.
It'll feel soft and pliable again after it 'sweats' for anywhere from a few hours to half a day depending on how dry it is when you start the process. you pull it out, let it dry again and repeat until it no longer is drawing out moisture from center -- at this point some 2-way humidity packs are all you need to control moisture content of the buds. 55-62% 2-way humidity packs will work well for this, just as they do for cigars in a humidor.
18C with 35% RH should not take long to dry. Slower is better, but this should take at least 5-7 days... the RH is quite low, so that will speed up the process a bit.
you can also weight some test nugs.. when near 20-25% of original mass, it's "close" ...
or weight the whole thing... smaller buds get there faster with higher surface to volume ratios.