Cal. Rev. & Tax. Code § 4142

Where delinquent taxes are being paid in installments, and a lien is sought to be satisfied and removed under this chapter, there shall be credited on the amount required to satisfy and remove the lien an amount which bears the same proportion to the amount which has been paid in installments on the whole assessment as the amount required to satisfy and remove the lien, excepting the redemption fee, bears to the amount necessary to redeem the whole assessment, excepting the redemption fee.