MyFixedPoint

I am trying to shift about /some/ of the material in inventory a to stack with the items in inventory b.
Sadly, both TransferItemTo and TransferItemFrom require a MyFixedPoint to do this, and I can't initialize or convert to one.
So how the heck do you actually transfer any quantity other than everything from one inventory to another using ingame scripts now?
I mean I have this distinct recollection of being able to do it before.