[natebrews]

Mostly tears and regret. 

But seriously, I just have some marks that I hammered into the side of the kettle at the gallon marks so I can look and see.  I put them on 2 sides so I can average if things are crooked. 

All in all, I don't worry about a couple points.  I doubt that most people could taste the difference in a modest difference (2 or 3 gravity points) in most normal strength beers.

[802Chris]

Boiloff rate truly is a little bit variable.  When I have 20-30 minutes left in the boil, I check the volume and how close it is to what I expected.  If too low then I consider whether to add more water to allow a longer boil or whether to start adding my late hops right away and shorten the boil.  And vice-versa, sometimes I have to plan for a longer boil.  In either case it doesn't affect the bittering significantly in most cases, but could affect timing of the late hops.

+1 here. Had to do it yesterday in fact when I undershot OG by over sparging. I also noticed a big swing in boil off rate my last few brews, now that the temperature is lower along with the humidity (Vermont)

[The Beerery]

I boil inside in my brewery which is always 66f and 34% humitidy(it has its own thermostat/huminidfier). I also use a special cut lid, and the same pid power on my element every time.  99 brews out of 100 I am within 2oz of volume and sub 1 sg point. So it is possible. Just takes some work!

[Phil_M]

Yeah, my old electric system that I used in the garage had no issues with repeatable boils. But it was out of the wind, and at least partially climate controlled...but lack of a decent ventilation meant it got damp...and the damp was bad for the car tools, let alone the structure.