Which method more accurate for calculating the paments?

Which method is more accurate for calculating the monthly payment?
using the updated schedule and multiplying the progress %age of each activity by its cost ?
or
using the detailed B.O.Q and multiplying the used quantities by its cost ?

i feel that if the updated %ages where not accurate the payment wouldn’t be also BUT calculating the quantities used every month would be more accurate

I just need to hear from you ...