I have chosen to pay interest annually on my I Bonds rather than paying it all at the time of redemption. To keep it simple I just take the difference between the current value and the value at the previous year end. I know this is probably a little understated due to timing of the interest being calculated & applied. Plus the fact that they don’t include the last 3 months of interest. Does accuracy really matter? When the bond matures or is redeemed the taxable interest gets trued up any way.