Separating fill areas

Hi,
I am new to the program, just working through my first take off. I am wondering how to go about figuring out a cut/ fill volume for only greenspace. The issue is the on site soil is unsuitable to be used for any areas other than greenspace. I would like to break down the greenspace balancing to use these soils to fill where needed. Basically, I would need to calculate the difference for all the greenspace from the existing elevations to proposed, I'm just not sure how best to do it.
Thanks