How Far Down Can I grind the seat with my reseating tool?

Seems to be some flaws in the brass of this tap (faucet?). It has fine scribbly black lines crossing from the inner of the pipe to the outside. I guess that's why it still drips a bit even after a number of reseating attempts and many new different washers.
Is this something others have seen? It's a flaw in the casting or it's something that is kinda 'working' even now and will get worse?
How far down can I grind without creating bigger problems?

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