local requirement for vegetated openspace < openspace

The regulation for vegetated open space is probably the more appropriate one to use here. The intent of setbacks was originally to reduce the chance of fire from spreading between buildings, and later became a way to influence light, density, massing, street scape, etc. So even though setbacks do create a quantity of open space, if there is another regulation that specifically address the quantity of vegetated space i would use that since that's my interpretation of what the credit is trying to address.