I can't tell where their data comes from, and they don't cite it.

The 10% number was not meant to be real, i just was giving an example :)

Real is much harder.

4% is not a horrible guess from as best i can calculate (but see below because this page has some crazy claims). Studies suggest that wood RH tracks RH pretty closely, slowing down with depth. Transport also appears to depends on temperature, independent of humidity itself. But if you assume it's going to track RH closely and throw out the rest, you can just assume the wood will always fall within the EMC range for the RH range.

If you look at

https://www.fpl.fs.usda.gov/documnts/fplgtr/fplgtr282/chapte...

You can see that between 30-60% RH, you really don't get more than like a 7% span (i'm eyeballing it) of EMC that the wood could vary around at any temperatures likely to exist in your house.

So 4% is probably not a horrible guess.

However,the site you link to says some very wrong things, interestingly:

"Temperature Has No Significant Effect on Wood MC"

This is 100% wrong, in more ways than one.

First actually even wrong if you ignore humidity entirely, because studies suggest wood moisture transport changes at high/low temperatures, even ignoring humidity. The exact mechanisms are not pinpointed (AFAICT from skimming), but that's what real data says.

Second, the temperature affects the EMC (and relative humidity).

It's very weird for them to go on and on about how humidity affects would but then say temperature doesn't matter at at all.

You can't actually separate these things, and say humidity level matters but temperature doesn't, because they are linked.

If you want real data/simulations to try to figure out more, here's some references - i didn't read all of them, busy morning, but i did at least look at most of them.

https://www.sciencedirect.com/science/article/abs/pii/S12962...

https://gupea.ub.gu.se/handle/2077/54179

https://www.mdpi.com/2076-3263/8/10/378

https://pmc.ncbi.nlm.nih.gov/articles/PMC8320951/