Im no expert at tiling but when i do carpentry im often cutting and scribing
Wood to different materials. I guess a chippys way will work in this instance.
Get yourself a a set of large compasses depending on size of tiles your cutting.
Put a pencil in them by what ever means possible aslong as when you do the tip touches
The point when shut.
Now put the point at the base of what your scribing to and the pencil 90 degrees plus a spacer
Thickness less from the tile next to it. Leave them a side for a sec
now get a fresh tile and put the point on the left side and mark a small line onto the tile.
Square that line straight up with a pencil.
Now place that fresh tile on top of the tile to the right.
Open the compasses out so the point is on the shape to be scribed eg toilet base
And pencil on the square line you marked earlier.
Then simply keep it 90 degrees to the toilt base and tile to the right and scribe it. Now you have the perfect scribe.
Explanation is long winded but not in practice.
Hope this helps