75% monitor match (16:9) is the same FOV/Sens scaling as Battlefield's Uniform Soldier Aiming with 1.33 coefficient and 1 zoom sensitivity for all guns. So if that's what you used in Battlefield, then you should convert 75% to Fortnite as it will be identical to Battlefield.
If you are expecting to use the same amount of mousepad space as Battlefield's hipfire, then your assumption is incorrect and could be why you are thinking it is not feeling right. Since Fortnite has a low FOV, it's going to require more mousepad space, similar to when your aiming down sights in Battlefield.