Though the Onderdonk house itself was at the eastern corner of the property, as per the Chester Wolverton Atlas of 1891, the estate comprised the entire 19 acre triangle bounded by Hempstead Ave, Hempstead Turnpike and Westminster Rd. Soon thereafter, LIRR President Austin Corbin purchased most of the property to lay a new rail line from Valley Stream to Mineola.
Since I didn't know at what angle the sketch was drawn, I had to resort to the Fowler & Bulger birdseye view of Hempstead below from 1876 to recreate the "now" photo.
The birdseye view above shows both the Bedell and Onderdonk houses, with the present-day Hempstead Tpke-Front St. running NW-SE and Hempstead Ave-Fulton St. running SW-NE. The Bedell House is just right of center while the Onderdonk House is at the far left. From the sketch, a porch exists on the near side of the Onderdonk House, while the far side of the house is obscured. The rear of the house has an extension that runs lengthwise, so it can't be that the Brooklyn Eagle sketch was rendered from the rear of the house. It's clear then, that the near porch in the birdseye view is the porch on the left side of the sketch, which was drawn from the street corner facing west - hence the angle provided in the "now" photo. As you can see below, the stately home of Henry M. Onderdonk has been replaced by a different sort of "castle".