I’ll second what @AdamsLeatherWorks said, and this may be helpful to some other folks too:
I mounted the feet of my Onefinity on 3/4" MDF blocks, making it the same height/level as my wasteboard. I was able to engrave a line around the perimeter of the maximum cutting area with a V-bit (so the tip of the V bit was 3/4" below the level of the feet). I also machined the whole spoilboard with the CNC in this configuration, so I was able to drill pockets/holes at least 3/4" below the feet level.
The main thing to keep in mind is the endmill length. The inexpensive pack of 10 1/8" diameter endmills that you can get on Amazon are only 1.5 inches (38.5mm) in length, and those are too short for this job. With the Z Slider mounted as far down as possible, you have about ~1" (~25.4mm) gap between the bottom of the router collet nut and the mounting plane of the Y Rail feet. You also need about 3/4" (19.05mm) of shank insertion length into the router chuck to fully engage the collet. So those details should help you choose the minimum bit length to do the job you want to do.
FWIW, when I flattened my spoilboard, I had to use an extension adapter since all of my spoilboard bits were just a bit too short. I used the Amana 47642 1/4" to 1/4" extension adapter and it was fantastic! Very little noticeable runout and my spoilboard was quite flat after a single pass (about .02" to 0.06"). I won’t use it often, but it’s a nice addition to the arsenal.
Cheers!