Hi again! :D Hm. If you wanted to get very technical you’d set up a 3D scene, say in Blender, and place a character dead center and make the camera isometric, that should give you the correct measurements. Or copy any isometric sprites from gameboy advance games. My sprites are usually so small and so stylized that it’s enough to make their feet align to the diagonals.
There is some foreshortening, but it’s the same amount you find in overhead sprites (for example pokemon sprites). In fact, the way for me to check if I’m doing a good job is to create a turnaround animation of a character in all 8 directions and play it continuously until I spot most of the mistakes.
Thanks for the question! I make them pixel by pixel. Now I know this sounds masochistic, and it is. But it also easier than shrinking high-rez art into pixelart and then trying to fix the ugly mess. Get into the habit of working at the actual resolution!