Enthusiastic about validating algorithm for calculating hashrate from stratum messages.

From my understanding following information is required:
start_time , time when miner began calculating
end_time, time when share from miner was discovered
issue, set by the mining pool

Method for calculating hashrate(discovered on-line) is:
hashrate = issue * 2**32 / (end_time - start_time)

Instance:
issue is 131072 from {"id":null,"technique":"mining.set_difficulty","params":[131072]}
(end_time - start_time) is 8.844114780426025 from one actual check instance

Calculation:
131072 * 2**32 / 8.844114780426025
131072 * 4294967296 / 8.844114780426025
562949953421312 / 8.844114780426025
Hashrate: 63652492917351.58 or round 63 Th/s

Questions:

1. Is that this appropriate ?
2. What’s 2**32 ? (suspect that it’s related to nonce in block header)