How is the one-third calculated when a specific item bequeathed dies before the testator?

Primary text

If the slave dies before the testator, the estate is valued on the day of the testator's death without including the slave. The slave is then valued hypothetically as if he were alive. If the value, when subtracted from the one-third entitlement, leaves a remainder, that remainder goes to Amr. If no remainder is left, Amr's bequest is void.