Ideally: run another node with appropriate settings, parse the result of getblocktemplate to compute the fees over an extended period.

My estimation is less accurate than that: it's based on a relatively medium sample size (several dozens with these settings over the past days). This can only illustrate what you could get these past days (and maybe detect a trend too). If one large pool upgrades its configuration and starts confirming the transactions currently waiting this number might go down. On the other side if one service starts to generate large numbers of transactions with fees just below what most of the miners accept (accepting a delay until a minority of miners confirm it or the txs age enough), this number will go up.