i think if you try to pay in advance the they will rig the bill next time.
This time will pay no advance . So i will pay Rs 864.
i
- so the bill was Rs 858.07
- I payed Rs 1000 . in addition to bill Rs 858.07, some money was added as advance payment.

- next month bill came Rs 1008.08
- I again paid Rs 1000.

- Next time bill was Rs. 1005.33 which is very close to Last payment Rs 1000
- this time I paid Rs 900 suspecting next bill we be close Rs 900

- I was right "Total charge" is Rs 894.93 which quite close to last payment of Rs 900

This time will pay no advance . So i will pay Rs 864.
i