As for heights though, it is very easy to estimate them when the aircraft in flight is sufficiently close to the building and to the power poles to be able to compare the dimensions directly. No experience is necessary.
Took me quite a long time in aviation before I could estimate altitudes, even with nearby reference. Not saying you're wrong, but not saying you're right, either.
As to how low before blowing things around, it depends on the type of helicopter, its airspeed (lower speed=more downward blow, simply put) and the objects blown over. (weight/dimension of these.)
In our neck of the woods it's illegal to cause stock to stampede at worst, and damned inconsiderate at best. It does sound like he was a bit stuck for the most obvious or best approach path (they can hover straight down to a landing, but it's very dangerous to do that, and generally approach at anything between about a 3 and a thirty degree angle) and unfortunately, best on the day might have been over your house.
I totally agree with the others above, you should try and contact the operator to express your concern, and if reasonable attempts to do that fails, complain to the officials.