In order to calculate the Ether sell-price threshold the application inquires all relevant parameters from several sources once per hour and stores them in a database. On every user request to calculate a new threshold value the application uses the last or the last six records stored and the following formula (see also page Profitablity):
hRig/hE • Δt/ΔtB • gB • KE = costsPerTimeUnit • Δt
⇔ KE = costsPerTimeUnit • ΔtB • hE / (hRig • gB)
From my physics studies many years ago I remember that that every measured value is useless without an information on its uncertainty. Therefore, the uncertainty of the threshold is deduced from the fluctuations of the various parameter values over time. So, the uncertainty of the threshold value is mainly determined by the total hashrate of the Ethereum network hE, the block creation time ΔtB, and the reward paid for one mined block gB, assuming that the hashrate of the rig hRig is nearly constant.
Currently, the last six measurements - i. e. six hours - are used to calculate all mean values and uncertainties.