As I understand it (and I could be wrong here, this is the Treasury after all, and any correction is welcome) is that bonds purchased by April 30 will pay the composite rate (0 base and 3.06 CPI) for six months, after which it will reset:
The semiannual inflation rate is determined each May 1 and November 1. Each semiannual inflation rate applies to all outstanding I Bonds for six months.
Fixed rates and semiannual inflation rates are combined to determine composite earnings rates. An I Bond's composite earnings rate changes every six months after its issue date. For example, the earnings rate for an I bond issued in March 1999 changes every March and September.
If my understanding is correct, i.e., that the rate as up to April 30 is good for six months, then even a reset base and CPI of zero for the next six months would be an annual rate of 1.53