Sum up Datediff based on filter

Hi,

hope someone can give me a hint for a problem I'm thinking about for a while now.

I have Sales with the Date of Sales and who sold it:

Table Sales:

Date | Person | Amount

I also have the date when the Person entered a new stage of experience:

Table Person:

Person | EndOfStage1 | EndOfStage2 | EndofStage3 | EntryDate | Manager

Now I need to calculate the Amount per day in a certain stage for a selectable time period. The result should be in total or by Manager.

So for Stage1 I need to sum up the amount of sales made by any Person in Stage 1 during the time of the sale

divided by the sum of days all people spend in Stage1 during the selected Period. 

I especially have problems with the sum of days, because it is not just difference between EntryDate and EndOfStage1. For example when the upper bound of the date filter is set in the middle of these two dates, then the diff between EntryDate and the upper bound should only be counted. 

Also the calculation has to be made for each person and after that summed up to the respective context (eg Manager).

Any clues on this?

Best Regards and many thanks