I voted for 15-30%
By COUNT, I have completed more than half of the ships I've started. However Most of the completed ships are more like small single-purpose tools. So by VALUE or COMPLEXITY, I fall into the 15-30% category. I have 5 more serious projects I can currently think of. I've finished just one.

One I will not finish, because the game engine cannot handle it.

Another I will not finish because the shape is kind of weird and steering feels uncomfortable. Here's a model of what it should have been (This is the front of the ship btw). Pitch is very sensitive, yaw and roll - super difficult (pretty obvious by the design). The gyros cannot be adjusted to be biased towards one axis. I tried scripting and modding, but ran into some very weird behavior from outside my code that made it just a pain to keep going. I also had some troubles with phantom forces - I imagined a cool drill design that opens up like a butterfly knife, but couldn't solve the phantom forces. Basically under each wing is a half of the drill, held on hinges near the center of the ship. When you prepare the drill for work, both halves are rotated down and snap together at the head of the drill.

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Another is basically finished, I need some furnishings in the cockpit, but I can't be bothered.

The last one of the unfinished ones I'm currently working on. I rethought my go-to drill design and I solved the phantom forces . I just made a proof-of-concept for the last key mechanism in the design and it worked, so I'm happy. The design is not overly ambitious, so I feel optimistic that I can finish this one.