A Summary field allows you to total, average, count fields (and other options). You can create a report that sub-summarizes by a field. Putting the summary field in the area that sub-summarizes by month will give you the month totals. If the field is set up for six months, then it will sub-summarize by six months.
You can use the summary field in a calculation by using the GetSummary function. For example,
GetSummary ( <summary field> ; <break field> ) / <number of months>
... will give you a total summarized by the break field (month interval) divided by the number of months to get a monthly consumption number.
This should get you pointed in the right direction.
If you need clarification for any of the above steps, please let me know.
I think it would be rather easy to setup a field to select the number of months you want the consumption calculated for, use this value in a calculation that counts back the selected number of months, and then use that calculation in a relationship to the Stock table, only relating records where the 'stock out' data entry date is greater than or equal to that date. Sounds like you are close to this already, you just need the calculated field and another relationship based on it.
Thanks chaps, I've kind of come up with an amalgamated solution.