You can definitely get to 20% without much trouble, maybe even 30-50%, if you do some freezing tricks. Though what such percentages mean is highly subjective.
I am thinking if an ideal butter croissant has some flaky fluffiness (perhaps if we define it as "trapped volume" between flakes), and we define this ideal flakiness to be 100%, then you can extremely easily get to 20% with just olive oil. Frankly I think you might even get close to 50% (defined in this way) provided you also start with a trustworthy recipe by mass and that aims for proper hydration (e.g. https://www.seriouseats.com/croissants-recipe-11863500) and work quickly with lots of chilling.
Just, subjectively, you might realize that 20-50%, defined this way, isn't much like a proper French croissant, and is more like a cheap doughy supermarket chain croissant—which I do still frankly enjoy sometimes anyway!
I am thinking if an ideal butter croissant has some flaky fluffiness (perhaps if we define it as "trapped volume" between flakes), and we define this ideal flakiness to be 100%, then you can extremely easily get to 20% with just olive oil. Frankly I think you might even get close to 50% (defined in this way) provided you also start with a trustworthy recipe by mass and that aims for proper hydration (e.g. https://www.seriouseats.com/croissants-recipe-11863500) and work quickly with lots of chilling.
Just, subjectively, you might realize that 20-50%, defined this way, isn't much like a proper French croissant, and is more like a cheap doughy supermarket chain croissant—which I do still frankly enjoy sometimes anyway!