I should have expected someone would ask me that… uh-oh!
I’m pretty sure I got the rack and pinion off Thingiverse somewhere (probably here… if so, "Thanks, @theodleif) and simply used his rack STL (not sure where I got the matching pinion STL, however) to build the Phlatprinter-inspired machine in 2017. I didn’t use Onshape then, however, and it wasn’t until 2020 when I did the R&P MPCNC that I had started using Onshape. I suspect I simply imported the STL rack mesh into Onshape and picked off the points for one tooth section. Again I’m not sure where I got the matching pinion gear… Greg’s extruder, maybe? 
The Thingiverse link above however have a “customizer” (that I can’t get to work) but there’s also a SCAD file to download, called “rack_herringbone.scad”. If you are familiar with OpenSCAD I’ll bet it’ll lead you to a gear library somewhere that will answer your question.
You’ve got my curiosity flowing now, Jim. I want to play with this some more… but a print just finished and I don’t want to slow progress on my machine mock-up.
Later.
EDIT: You might want to take a look at this Github link and download “gears.scad”. Supposedly does herringbone racks and gears.