Later (CB, DB, DBD) eccentric adjust engines have a limited adjustment range which in that case could mean a slightly longer or shorter push rod becomes necessary to get a good range of tappet adjustment. ... Also, mid point of the eccentrics rotation is a nice target but not absolutely necessary.
I had to make my own pushrods for my Ariel and the photograph shows the adjustable faux pushrod I made to determine the length I needed
The Ariel engine has adjustments at both ends so I set them mid-range, measured the spacing with a ruler, and made the faux pushrod short enough that at least a few threads of the fine-thread bolt (¼-28) would be exposed to allow for adjustment in both directions to compensate for the relatively inaccurate measurement of the ruler. Once installed, I turned the bolt to give the faux pushrod a fit that seemed appropriate and then accurately measured the total length to use for making the actual pushrods.