calculate the snapshot blcok number

Yes, if you wish you can make the same calculation. But I have also calculated it:
Here is the note from the original announcement: TECHNICAL NOTES: To determine the exact block of the snapshot, the block id of the first block produced after 0:00 UTC September 16 will be divided by the number of blocks produced from 0:00 UTC September 1st through 0:00 UTC September 16th, and the remainder (mod) of that division will represent the block number (from this date range) that the snapshot will be taken.
And here is the calculation:
determining_block=9672114 first block after 9/16 UTC 0
http://www.cryptofresh.com/b/9672114
blockid of determining_block = 009395b262d8ec93b6c8c71d8560539f40a6c1ad

first_block= 9243083 (9/1 UTC 0)
http://www.cryptofresh.com/b/9243083

blocks_produced = determining_block - first_block = 429031
sharedrop_offset = blockid of determining_block mod blocks_produced = 48517

sharedrop_block = first_block + sharedrop_offset = 9243083 + 48517 =
9291600
http://www.cryptofresh.com/b/9291600
www.cryptofresh.com
9672114 | BitShares Block
www.cryptofresh.com
9243083 | BitShares Block
www.cryptofresh.com
9291600 | BitShares Block