I have a 30-qt turkey fryer kettle and find that I'm a little low on my batch volume towards the end of the boil. So lately I've been using extra the runnings from my sparge, saving it, and adding it towards the end of the boil. This has two effects: (1) brings my volume up to 5.5-6 gallons so I have a full 5 gallons after racking off trub in the primary and (2) brings my OG up a couple points which I've been tending to miss. I'm just starting with the all-grain and thought this was a brilliant idea until I started thinking of why we do 60 minute boils. Is there any reason this could be bad? I bring the wort back to a boil before chilling to make sure there's no bugs left. Also... should the "downtime" while the wort isn't boiling not be included in the total boil time?
To compensate for boil off volume, just watch what you're boiling off after a few times and there you go. Add that to your recipe when you do it in the first place. I started brewing in doors, in my kitchen, and I was boiling off about a gallon an hour. I had no idea it would be any different when I moved outside, on a big ass burner. After overshooting my post boil gravity, I realized I boiled off "too much". I now boil off about 1.5 gallons an hour, outside, so I take that in to account to my pre-boil volume.
Maybe I'm missing something, but I don't know how your OG could go up with adding saved up runnings. Even if you collect all of your wort after sparging into one big bucket and only boil up 5 gallons of it and that comes to 4.5 gallons of 1.056 wort and you add 1 gallon of 1.044 (~pre boil gravity), you're gravity is going to be 1.054. Still less than what you've boiled down to.
In a 30 qt pot (7.5 gallons), I've been able to get 7 gallons (for 90 minute boils) to boil on my stove. Yes, I have to watch it for boil overs, but I've been able to do it. Either way, you shouldn't be boiling off 1.5 to 2 gallons an hour inside on a stove. I have one of those power plus burners, and I'm only burning off a gallon an hour in there. So you should be able to do a 6 to 6.5 gallon pre boil volume in your recipe and should be ok.
If you're brewing outside and you're boiling off 2 gallons an hour (crazy, but I've heard of it), lower the fire power on that rocket ship that's cooking your wort.
What I did was, I took the same pot and boiled 6 gallons of water inside, and 6 gallons of water outside. That gave me my different boil rates. This might change with the sugary wort and other factors like humidity, but that's going to get you real close. Once you figure out your boil off rate, you'll be able to formulate recipes better.