Idh lā tukālu al-dimāʾ does not describe literally measuring blood like grain. Tukālu is an imperfect passive verb; it places blood in the conceptual position of something accounted for or measured. The commentary links this image to requital and the settlement of blood claims.
The figurative force lies in compressing violent, difficult claims into a scene of measurement. Read the lexical meaning (“to measure out”) alongside the intended sense (“to account for and settle bloodshed”) instead of confusing the two.