The checkerboard premise of 50/50 is somewhat arbitrary, but feels like a good subdivision. Latitude is always more or less 69 miles per degree, while longitude gets smaller as you move from the equator to the poles.
So, near the equator, the proposal is for 69×69 mile (111x111km) preserves, about 3 million acres each. For reference, Yellowstone park is about 2.2 million acres. In some areas like deserts or open ocean, it may be desirable to expand to larger checkerboard squares, but it is important to keep representative ecologies in preserve areas (i.e. not make squares so large that entire ecosystem types are contained within a single developed square and not significantly represented in a preserve area.) Of course, near the poles, especially above 60 degrees latitude, multiple degrees of longitude would be used to avoid the checkerboard sections getting too thin.
In many places, it will make sense to bend the lines to put unique terrain in a preserve area, or to keep urban development intact in exchange for preserving additional nearby areas. Especially at the corners, provisions would be made for efficient transit roads between developed areas. Provisions should also be made for transit of wildlife between preserved areas. Generally, a one mile wide transit space between preserved areas should be connected at at least 1/2 of the corners – picture an elevated highway, power lines and pipelines crossing this area – still counted as developed land for the balance, but allowing free transit for wildlife between the adjacent preserves.
Why not just stripes? Mostly, transit at the corners would be less disruptive than 69 mile bridges. With a checkerboard system, surface and air travel can be restricted to the corners and out of the preserved areas.
When choosing areas for exchange, draw triangles from the corners to the the center of each checkerboard square. For each acre of land in a preserve square that is going to continue to be used for development, at least one acre in the adjacent triangle of the developed square must be preserved. All preserved land for a given square must be contiguous, including exchange land in the developed square.
Why 111 kilometer squares? Based on human development to-date, a 2 to 3 million acre area is sufficient to encompass most developed cities and their surrounding areas, it’s not too restrictive, while still remaining small enough to not completely wipe out unique habitats, or human cultural heritage. Most (though, not all) countries encompass far more than 3 million acres, and can accommodate the preservation subdivision without being completely deprived of access to their various terrains for development. “Giving up” 50% of your available land for development is a radical thought, but, at least for competitive purposes, if everybody, everywhere transforms 50% of their land (and ocean) into preserves, nobody would have an unfair advantage, and we all would reap the benefits of living on a planet with functional ecosystems.