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.