The numbers seem overturned tbh. You essentially have a 55% chance of getting an attachment. So let's say you bring 3 for example. You'd have a 9.11% chance of failing all three. Which means a 90.89% chance of successfully getting at least one. As well, on the scale of a castle the difference of even 8 blocks really isn't that much. Since the idea is to cross the gap and then pretty much use ladders up the wall, you only really need to hit the wall.
The picture I attached would should the different levels (red is perfect, blue is second, green third, pink fourth). In the grand scheme of just trying to cross the gap it seems quite powerful. https://gyazo.com/ccfd5214e875f754654375f9aa63bc22
As well, just for an example, the bridge connecting the Krag to the mainland is 76 blocks (which is the shortest distance between the Krag -> Land & it stops on two separate stone pillars). I know mineman scale isn't perfect but each block being a meter, 76 blocks is about 83 yards for us Americans (8/10 the length of a football field). You would need an extreme amount of force to launch a bridge made of steel and wood across that large of a gap and then into a castle/mountain of stone. Not to mention the amount of tension to keep the bridge tight enough to walk across.
I think it's a good idea, but extremely overpowered.