For setting the ruin area, it would be best if we could find an information source of where to where was actually owned by McAndrews among the eastern parcel. In the case of Cornish estate, unfortunately, there were no reliable record of where to where was a property of Cornish. So I just set the ruin area so that most of the buildings would be included (still, there are some building-ruins outside of area that I set). For Northgate Farm, images of USGS 3D Elevation Program worked well to estimate the area of Farm.

It's always better to think that 'what' include 'what'. In this case, the Park (Oscawana Park) > Area in different use (McAndrews Estate) > Buildings or structures (each buildings in estate). Please once take a look at how I and other people did for Stern-Cornish Estate Ruins in Hudson Highlands State Park. The State Park includes the two 'ruin areas', 'Stern-Cornish Estate Ruins' and 'Northgate Farm', which further include 'building ruins' like mansion, 'greenhouse' or 'barn' etc, etc, respectively.

In my understanding, both the 'Oscawana Island Nature Preserve' and 'McAndrews Estate' are parts of larger criteria ’Oscawana Park'. In such a case, At first the both the area should be marked as single item 'Oscawana Park', and then corresponding areas could be overlaid with different 'Features' with corresponding names, respectively. For example, the 'McAndrews Estate' can be marked as a 'Features' of 'Ruin' area overlaid onto the Oscawana Park.