I assume you have read this:
http://salemwiki.info/index.php/SoakAnd I guess that's all to say.
Stone hedge has 15 soak.
If you have 26 phlegm then your base damage to structures is sqrt (26/10)*10 = sqrt(2.6)*10 = 16.12
From base damage soak is substracted. 16-15 = 1
That's the damage you will deal to stone hedge at 26 phlegm.
If you want to count hitpoints of a building then you have to first determine what soak value it has (ie when you start dealing any damage). Then, knowing soak value you will know how much damage you deal. Knowing this and seeing how many hits you need to destroy it you can guess its health