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.