Calculation of Open space

I have a leed boundary of 100 acre out of which 40 acre is green field and 60 acre is previously developed. For calculation of protect or restore habitat, I have set aside 18 Acre of previously developed area (30%) for my development foot print. The remaining development footprint available is now 42 acre. For creating the open space how to calculate the 30% requirement. Is the 30% shall be from the remaining 42 Acre or shall I use the 18 Acre set for restoration in the previous credit to be used for open space also