Computation of VAT and NonVAT on Sales Invoice

hi im from Philippines, our Vat and Vat Exempt Computation is like this:
VAT Amount / 1.12 x .12
VAT Exempt amount x 1%

Example
Total Amount of VAT & Vat Exempt
17838.5 + 5872.5 = 23,711

the computation of 1% Withholding tax in manager.io is 237.11

here in the philippines the computation is like this:

17838.5 / 1.12 = 15927.23 Exclusive of VAT
15,927.23 x .12 = 1911.27 VAT
5872.5(Vat Exempt) + 15,927.23(Exclusive) =21799.73 Total Exclusive of VAT
21799.73 x 1%(Withholding) = 218.00
17838.5 + 5872.5 = 23711 - 218(Withholding)
Net Amount = 23,493.00

in the system, it computes the VAT and Vat Exempt x 1%.

thats why im having trouble in the computation. Is there any way to customize the computation of the system? thank you and God Bless!