The PSoC4 has no external low frequency oscillator. You can only use an external oscillator, and route its (TTL or CMOS compatible signal) in via a GPIO. Or you use a high frequency crystal, and tune the ILO with this. There is an AppNote for PSoC3/5 explaining this.
According to ap note that approach only good to +/- 7.5%, so will not meet
your target of 1%.
Another approach is at production test, use a cal routine -