Hi Steve, thanks for your reply,

I'm kind of in the same boat. I have to go also back to an FM5 application
and I'm a little rusty...

I'm completely aware that it will not be so easy, but then, that's the
challenge.

What I have so far to control the inventory is: 
+ Initial on hand qty
- inventory transfer
- scrapped qty
- lost/damaged 
+ total received
+ total produced
- returned to Vendor
- total consumed
This should give the total inventory on item level.
+ total pending PO
- total pending SO
+ inventory transit
+ total backorder PO
- total backorder SO
This should give the total available on item level.

I think I cover pretty much every possibility (hope)
I think I have to play with dates and some operators within or between the
above fields to end up with a 'yes' or 'no'.

According to you it is doable. 
So I'm confident I'm not chasing something impossible.
I'm looking at an amount of trial and error and testing...

Tx.

John