Latest NMRA O-gauge Standards

I wouldn't bother buying one for 3 rail use. It won't tell you anything useful about curves and clearances. You'll do better reading the standards, and following some basic rules for track center placement.

Straight track is simple, I use 4" centers, but you could go as close as 3.5". Curves are all about circle diameter, car length, and spacing. Once again I use 4" and all my curves are 072 or larger. I have run into a few long cars that cause problems passing on those curves. To be completely safe with something like a Big Boy, curves should go out to about 5.5" centers. 

If you have questions about specific situations, just ask them, and the guys will answer.

BTW, I've been an NMRA member for more than 40 years, but 3 rail marches to its own beat. I don't own one.

I used a chunk of rough cut 2x4 (larger than a finished 2x4 which looses some size to finishing) to space for 0-27 and my walls (0-27 needs more clearence).  For curve to curve, measure longest car stringline overhang, and engine with largest pilot overhang and/or cab roof overhang/kickout (use largest of those 2 measurements) add those 2 dimensions + 1/4"-1/2" for clearence... a ½" imo because cars do sway,  and  so the top's static measurement isn't "perfect" and is more than downow at the frame.

I have no idea what "downow" is  .... ask google. They insist on it, so ask them. Ask your brother to ask them, and cousin Fred, and the neighbors . Maybe Google will actually do something about it  then .... oh, man; that's funny too. Google doesn't fix anything. They likely just point and laugh that we accepted a Linux derivative at all. 🤣