Altitude accuracy

Given the improvements in in Lat and Long what is the certainty of altitude accuracy? I have a specific requirement not only to have greater precision in lat and long but also height over time - ie subsidence.

I use the rule of thumb, and its x 2 times the horizontal value
But here is the faq
https://docs.emlid.com/reach/faq/

Hi many thanks for the reply - so much to read , so much to learn!