This can be tricky, as the tile, its depth, the texture of the surface, and the thickness of your tile mud, all play a part in the answer.
Most informed sources say that at first, it's all about experimenting to figure it out, based on the specific variables of the surface you are tiling. While that is a little vague, it isn't necessarily wrong. However, if you are tiling a wall, or a floor, there are factors that adjust the equation.
Even the smoothest concrete floor will have imperfections or areas that are different after the concrete settles. On the other hand, tiling a sheetrock wall is a smooth canvas for even distribution of tile mud. So, the surface doesn't change.
The best way is to:
Step 1: Measure the edge of the tile you intend to use. Use that as a metric for the absolute thickest trim you can use.
Step 2: Determine how much tile mud will be needed to affix it to the tiled area for the transition in question.
Step 3: Spread a layer of tile mud, less than the mud used on the tile itself, on the tile trim, and push it into place. If it is flush with the face of the tile, then you have used the proper amount. Tile mud doesn't dry quickly enough that if the edges aren't flush the trim can't be removed. A simple pull on the trim will free it from the wall or floor.
Step 4: If needed, add or subtract tile mud to compensate for flush continuity with the tiles around it.
Before you start tiling an area, knowing the thickness of the tiles, and the thickness of the trim, is critical for a smooth, clean look. A thin tile with a thick pencil liner trim will guarantee many toe stubbings in your future.