The block that will be the basis for the Altair snapshot is **6650475**.

The calculation is available at wolfram alpha

There are four steps to this calculation:

- Divide the block hash by 2^256. This gives us a number between 0 and 1 (in this case, approximately 0.8).
- Multiply that number by 28800, the number of blocks in the snapshot range. This gives us a number between 0 and 28800 (in our case, about 24104.5)
- Remove any digits after the decimal point
- Add the first block in the snapshot range, giving a value between 6626371 and 6655171. This gives us our final result of 6650475.

For the more technically inclined, the same logic is also implemented in python and javascript, and has given the same result.