Generally I've seen it rules that the overlap needs to cover >50% of the square to affect it. In which case you have the exact same coverage as before. But in this case I'd just rule that we shift the grid 45 degrees so it lines up again and move units to the closest square that aligns. I.e. don't use geometry to try to munchkin.
There's a bunch of grid-based shapes that pertain to the rules of 5e as well if you need that. Also 5e fun fact: a circle or radius affect is going to look like a square since diagonal distance is not accounted for, but in Pf2e it looks closer to an actual circle.
You’re welcome! For reference it’s a code block, all formatting goes out the window, returns are considered returns, and a monospaced font. You use three backticks (```) on a line above and below your “code” (you can technically specify the code type at the end of that first back tick line) and then go to town between them.
Feats are also an optional rule, but I've never heard of a table not using them. "Optional rule" in 5e is kind of like the term "theory" IRL, in that some really are optional and some are basically always used. I will admit that not all tables use the diagonal rule, though.
I always houserule that "circles are square", because it's so much quicker and easier. It gets a little extreme when you do it in 3D, but I don't overly care.
(For fun, a double diagonal move of 5 places you at 8.66 from the start, almost 75% further than an orthangol move, but still 5 for game purposes)