Might I suggest for the Reference Section this Apple Scion/Rootstock Selection calculator created by R.L. Perry and J. Hull of Michigan State and Jon Clements of the University of Massachusetts?
It helps you determine the spacing of apple trees in an orchard using multiple factors including scion vigor, rootstock vigor, soil type, irrigation, and management system. Created in 2012, it was designed for use in Michigan, but it should be relevant elsewhere. In addition to the calculator on the “Apple Scion/Rootstock Selection and Planning for Michigan” page, there is a link to a simplified version for mobile devices.
Adding some additional measure of productivity (or growth potential) such as annual or growing season precip in the absence of irrigation could be helpful.
I played with this one a couple years ago and the suggested spacing was pretty much what I had expected for the various root stock and variety combinations I have. I’ll probably use those figures when I begin planting out.
The Home Orchard Society has a page of Estimated Tree Vigor.
But it doesn’t match.
Spacing CAN also be determined by how much room you have and how much you are willing to prune. This youtube video talks about having many trees (apple in his case) in a very small area. He said you could prune any rootstock to keep the tree 8 foot or shorter. He was more into varieties that getting the most production from each tree. His comment that you are a backyard fruit tree grower not a commercial grower, you do not need tall or big trees.
True enough,but you’ll never keep seedling rootstocks small enough so you can plant just a yard apart and be successful…takes dwarfing roots to enable close plantings.