Use min/max displacement level in material’s settings. Hope it helps. 
First, chech your displacement map. If it only has black and white values, it is normal behaviour. Dome gray values (like a gradient) will smooth it out.
Secondly, maybe you need to subdivide your mesh into smaller pieces, could be helpfull.
Displacement in Corona has units relative to your scene units.
You can assign a defined displacement height in mts cms or whatever unit u use in apropiate material tab (Min Max values as maru said).
It helps a lot if you can put an image of your displacement map or material settings so we can help you better.
Where did you change that value? I don’t remember any percentage-driven setting for displacement. What version are you using?
