To be exact, you would need to guess 23 words and then calculate the 24th, as it is the checksum of the 23 previous words.
Wrong. The checksum isn't the 24th word. The checksum in a 24 word seed phrase is the last 8 bits. This means that the first 3 bits of the last word is determined randomly and the last 8 bits are the function the 256 bit entropy.
If you have 23 words and don't have the last word, there would be 8 possibilities for the last word.