Balanced field length of course is what V1 is predicated on. You can stop on a runway or be at 35 ft at the end. Most of the time you have additional runway so balanced field length isn't important. Taking off from MIA with 13,000 ft of runway we still called V1 as a decision speed but knew we had tons of runway left if we chose after V1 to stop. The FAA wouldn't like it but it would work just fine.