Scheduler Tool works but can it be simpler? Does anyone have a simpler scheduling formula?

Spreadsheet works but is too large.
Enter shipment lots in month, row 3 cell F3 onwards
150,000 units made pa, 1st shipment Jan 2013
Repeat shipment at cell A3 ie when 300,000 is reached (value can be changed as desired)
Automatic schedule re appears in row 4, into the future and each time a multiple of 300,000 is reached eg 300k, 600k, 900k etc
Now look at the number of lines it has taken (unhide the rows)
Does anyone have a scheduling formula that could replicate the spreadsheet? There must be a simpler way of doing this. Any help appreciated.



Chambo