So there are a couple of constraints here and the current solution seems like the best to me, but to put them out:

1) The game uses unforgiving comparison of position and the edge of the tile. So two of the edges allow tilling right on the edge, but the other two do not. Basically it's doing <= and > instead of <= and >=, although technically this is more annoying for them to fix than that because what it's doing is "given this point, what tile is under it?" so every point has to resolve to a single tile. Therefore every edge is assigned to one tile, and by parity two will end up on a given tile.
2) The actual till spacing allowed by the game is 1.25, but tiles are 4 by 4. So if I do a grid with exact spacing you end up with a grid that doesn't align with tiles, but shifts by 0.25 relative to the tiles every 3 points. This make for ugly spacing at best and inconsistent amounts of valid placements per tile at worst.
3) Farming mechanics actively discourage 4x4 with the overcrowding stressor, with 10 being the max before that kicks in.

My current solution is to use 4/3 or 1.33333 as the spacing. This means that the grid stays aligned with tiles (addresses #2) and doesn't shift relative to them as you move between tiles. This makes it pretty easy to get 3x3 with the rectangular grid, and feasible to get 3x4 with the hexagonal grid, which is enough to fit 10 crops on.

As I see it, this solution addresses (2), is more in line with (3) than using 1.25, but makes (1) more noticeable as there are always grid points on the edges. Unfortunately it's not really possible to both allow 4x4 and still address (2) in a generic way. Snapping Tills solves this by using mode-switching based on whether the hovered tile has adjacent soil tiles, which is something I could do but I'm not convinced is worth it (in terms of potential benefits vs likely confusion), if that makes sense.

but I want to plant forget-me-lots...QAQ