sale invoice UDF

Hi All,

I am implementing for a service industry where they provide printer service on total guarantee basis and bill only for printed copies. They provide for Govt sector and have hundreds of printers in each department.

The requirement is.. The sale invoice should get the previous sale invoice's closing reading as new sale invoice's starting reading.

For ex: this month's reading is: starting 100 and close reading is 150. so they bill for 50 copies.

for next month sale invoice, it should automatically show the starting reading as 150

Kindly suggest me how we can write the FMS