The reason I posted originally was because you were being misinformed. And still after I demonstrated with real-time, real-world data why using the network has rate is a flawed method, there are arguments.
There should be no requirement to convince anyone of math. My equation is derived from the average time required to find a block which I gleaned from the bitcoin wiki:
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How soon might I expect to generate a block?
(The eternal question.)
The average time to find a block can be approximated by calculating:
time = difficulty * 2**32 / hashrate
where difficulty is the current difficulty, hashrate is the number of hashes your miner calculates per second, and time is the average in seconds between the blocks you find.
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The network hashrate plays no part in this whatsoever. The network hashrate (for bitcoin) is derived from the rate at which the last 120 blocks (by default) are found as shown here in a snippet of code from Github:
98 "getnetworkhashps ( blocks height )\n"
99 "\nReturns the estimated network hashes per second based on the last n blocks.\n"
100 "Pass in [blocks] to override # of blocks, -1 specifies since last difficulty change.\n"
101 "Pass in [height] to estimate the network speed at the time when a certain block was found.\n"
102 "\nArguments:\n"
103 "1. blocks (numeric, optional, default=120) The number of blocks, or -1 for blocks since last difficulty change.\n"
104 "2. height (numeric, optional, default=-1) To estimate at the time of the given height.\n"
105 "\nResult:\n"
106 "x (numeric) Hashes per second estimated\n"
107 "\nExamples:\n"
108 + HelpExampleCli("getnetworkhashps", "")
109 + HelpExampleRpc("getnetworkhashps", "")
So if the network hashrate is steadily declining as it presently seems to be for bitcoin, the value will be skewed and not representative of the current network hashing rate. This is moot however since the current network hashing rate has little if anything to do with your probability of finding coins and you are only fighting against time and the current difficulty.