creating a forecast which automatically divide production into available weeks

I’ve trawled through the material I could find on forecasts, but I have not found a way to automatically create following forecast

Situation and problem description:

Once every third month (or so) we receive a forecast describing that we may deliver X pieces of item Y in a period from date A to date B

As an initial state I would like the production of the item to be equally divided into the weeks available, and I would like the values to be automatically updated if the user alter the dates (shrink or expand the period of production). The number of days between A and B may differ from one forecast to another.

Bad solutions:

Axapta provides 2 ways to define allocation: by period or key:

If I (when importing the forecast into the system) find the average amount per week and use this as Sales quantity with a period for 7 days in a forecast, we’ll not achieve the correct amount if the period is changed to less or more than the original.

The same issue arise if I use a key, since I have not found a way to make the percentage depending on the number of days available.

Have any of you guys/girls seen/solved this problem before and how ?