Cal. Rev. & Tax. Code § 4158

Where delinquent taxes are being paid in installments, and a parcel is separately redeemed under this chapter, there shall be credited on the amount necessary to redeem the parcel an amount which bears the same proportion to the total amount which has been paid in installments on the whole parcel as the redemption amount less the redemption fee on the parcel bears to the redemption amount less the redemption fee on the whole parcel.