For portfolio management I need the risk-free to compute the sharpe ratio.
I would like the use to rate on the 3 month treasury bill from the US. https://fred.stlouisfed.org/series/TB3MS#0
The data can be obtained via the link above, however I have some doubt about the exact calculation of the risk-free rate. My portfolio consist of the daily returns from 31/12/1989 till 31/12/2018, this is in total 29 years and 7309 daily observations
Do I take the average of the annual rates as a proxy? Or what is the best method?