Nobody can answer that question- aside from supercell - or maybe some player I have never met who may claim to know more...
Read this post =
http://forum.supercell.net/showthrea...e-Grand-Warden
Many smart players have replied... but generally- Weight is not always the same simple X Y Z thing... its impossible to know exactly anything depending on the general choices a player makes every last defense -could - weight a different amount at differrent stages of that base's life.
All you can do is extrapolate = based on the info provided... to know that =at worst - it will not be higher than the highest reported number; and at best - it will likely not be less than the lowest reported #...
If you can always three star players who are 10k heavier than your own personal weight = then it is safe to upgrade one xbow.. and after its finished see the new weight = make sure you can still 3 star above mirror... then maybe do another...
10k minimum; personally if I can not 3 star bases that are 40k heavier I will not upgrade defenses... as a 40/50k player I want to 3 star near max th10 accounts while bringing th8 less as a mirror...
each to their own... depends on how epic you want your clan war log to be.

