Hi thea carp,
We dug a bit the question with a colleague and this might help you:
If datetrunc('month',[date])=[Start Date] then -[volume]
datetrunc('month',[date])=[End Date] then [volume]
else 0 End
Then Sum() those on screen and add datetrunc('month',[Start Date]). Same thing goes for End date.
Amazing! Thank you so much for your fast reply! I'll give it a try.