First off, I would concern myself more with aiming for a post-boil gravity, not volume (but you may have also meant that). It's the gravity that's going to have a big impact on the character of the beer. Who cares if it's a few pints under or over in the end?
For my 60 minute addition, I don't make any adjustment. For my late additions, I adjust accordingly based on expected time left in the boil. So if you were extending the boil by 30 minutes, all late additions will be pushed back by the same 30 minutes to keep their "duration until the end" constant.
I do all 75 minute boils, even for recipes heavy with pilsner malt, mainly to better predict boil off. I add the "60 minute" hops 15 minutes in. If these go longer than 60, you're not going to significantly affect IBUs. When I'm half way through, ie 37 minutes-ish, I check my volume against what I would expect for half the boil off of the full boil duration. If I'm significantly under or over, I adjust my flame intensity to compensate, I don't just lengthen or shorten the boil.
If you are finding you routinely need to take a planned 60-minute boil to 120 minutes to achieve a volume, you have bigger problems with your sparge that need addressing first, IMO.
I hope that all makes sense. Feel free to ask follow up questions.
Eleventy-Three Brewing Co.
Kegged: Ten Downing ESB, Grizzly Grundle Pale Ale Remux, Eldred Rock Light Northern German Altbier, Unnamed Kölsch.
Bottled: Brooke's Bliss Irish Stout
Secondary: Who does that?
On Deck: German Pilsner