FIFO Inventory Formula Challenge

Twifoowrote, responding to my analogizing LET as a welcome variation (improvement) on helper columns,  My analysis of the LET syntax reveals that it defines a name for the value in an argument, then use such name in the main formula. If such value, which has now been defined as a name, won't be reused in the main formula, such definition becomes superfluous! 

And I fully agree. LET does in fact define a name for a value, often for the result of an independent calculation.

In doing that latter, it operates as an improvement on the less efficient, albeit probably easier-for-the-novice-to-follow use of helper columns. That was my only point; LET is a nice, and once grasped actually entirely intelligible, way to accomplish some remarkable combinations of calculations that hitherto were separated into multiple columns.