I always assumed he blew it up ONLY because he trusted the force but now I am not so sure.
shareThis movie is a work of fiction. Nothing was ever filmed or discussed about whether the Death Star would have blown up any other way
shareI believe your initial assumption is correct. I think if Luke had relied on the computer, he would have missed. Here's why:
Perhaps this is cheating, because it's not "in universe", but to some extent, I think this because it makes the story work. Luke's journey as a character involves becoming confident (traveling from childhood (adolescenthood?) to adulthood, or at least stepping that way), and involves following his father's footsteps by grasping his inner power: the Force. So, the story feels more complete if he learns to use his instincts and his inner strength. Because the story requires it, I say without it, he'd have missed.
But, if you're hankering for an in-world explanation, here goes... I think the reason the Red Squadron leader missed was because his targeting computer did the work. Not that Red Leader could have hit with the Force, but I think the computer can't make the shot. My theory is that something about the shape and position of the exhaust port befuddles a targeting computer. Perhaps this is a deliberate design choice, perhaps it's just a quirk, but I think it's basically like a printer that's out of alignment, or sound/video that isn't synced up. The computer thinks it's hitting the target, but it's not. Luke's computer would have been mis-calibrated as well and bunked up the shot.
Of course, it's all speculation. All we know is that, in the story of the film, Luke uses the Force and makes the shot. We don't know what "would have" happened otherwise.
For a final kick, though, how about Obi-Wan Kenobi encouraging Luke to use the Force. Maybe he knew Luke could do it with the Force. Maybe he knew he'd miss without it.
What's making you doubt your original assumption?