Measured a ton of scopes, and then verified what we thought against some other mounts, honestly. We then scraped every forum or post possible about any mounts that didn't work with any scopes. When there is something that doesn't work, there tends to be a thread about it somewhere.
We wanted enough space that anything would fit, with appropriate room to move at least the length of one pic section, but still fit between the bell of the scope and the ocular housing. There's a decent window, we found a spot in the middle that we liked. Same thing with our tube diameters, measure everything we can, figure out the "extreme spread" and make some fit and tolerance decisions on what we want to do.
And to be clear, we got it wrong (kind of) on the 34mm one-piece at first. At the time we rolled them out, they fit EVERYTHING, then the MK4HD dropped, with the longer teardrop shaped erector housing, so we lengthened that mount, so we had to alter the dimensions a bit. It creates less clearance on some scopes front to back, but it was available.
Doing everything you can to get things right at launch is obviously important, but being adaptable (as a company that makes accessories or items that have to integrate with other products) and willing to make mid-production updates is also hugely important.