When the number of residential buildings is unknown
If you don't yet have detailed plans which say how many units you're planning to build, you should use a default 70:30 ratio of ‘urban–developed land; sealed surface’ to "urban–vegetated garden" (i.e. draw on the intended work screen a block of "urban - developed land, sealed surface" for 70% of the area you're planning to build on, and then draw "urban - vegetated garden" over the remaining 30%).
This ratio:
does not account for access roads and public access space
may be adjusted for particularly high-density or low-density developments
(if you do use this ratio - or a variation thereof - you should make sure you record this in a comment and explain why you move away from the 70:30 ratio recommended by the Government).