How to calculate specific area?

Hi, I want to calculate area based on contour plot result (Figure A). In my case, I want to calculate the area of overflow (Figure B)

which bigger than 0.3. Here are some limitations that I use in my script.

h=dem
contour(x,y,zmax',[0.3 0.3]
contour(x,y,-dem',[0 0]