@Steve_Hall Wonderful diagrams! Generally speaking the motors should be about 1 foot out from the side of the cutting area. The dimension is correct in your drawing, but could be misleading if someone wanted a shorter bed. The default setting is 463mm (~18.25in) for the motor height, so you’re a little low.
As far as I know, no one has run bed accuracy tests on a stock size machine. I have tested as many of the linkage kits as I’ve been able to acquire. Full thread here, I’ve set the link to load at my post when I actually started running tests. That is, in case you haven’t already seen it 
You’d be surprised with the level of accuracy you can get with the 45 degree kit. I have a short top beam, which is physically 112" long, and I have reliable accuracy across much of the bottom of the bed. I know people have also dialed their machines in better than mine. I’m sure with the 120" top beam the new stock design is going to have, that would be pretty tight tolerances across the bottom of the bed.
I’m still very curious to see how @bar’s ring will compare to the 45 degree linkage 
But I digress, this is a thread about frame dimensions, not machine accuracy.