Subtracting from different worksheets & finding the "last" match in column

Good day to all,
I am new to Excel formulas/functions and am struggling to make my inventory system work (all while trying to learn Excel functions). Hoping someone can help. I will try to explain the attached workbook and what I need it to do:


The Workbook (5 worksheets):
1) Slug Inventory: This worksheet will contain the “real-time” inventory level by Slug #, as well as its MIN level.
2) Inventory Rec.: When we receive shipments of the Slugs we order, Shipping will input the inventory received into a Google Form, which downloads into this worksheet.
3) Slug Usage: When we use Slug inventory, team members will complete a different Google Form to track the inventory that was pulled, as well as the unused inventory that is put back on the shelf. This worksheet retains the data from that form.
4) Cell Locos: This worksheet needs to show which cell location each Slug # (and its quantity) is located at in our warehouse.
5) Workbook Functions: This is just a worksheet that I created to help and to keep track of functions used in this workbook.


Functions I need help with:
1) Subtracting from Different Worksheets: I cannot figure out how to subtract the “Slug Usage” in worksheet 3 from the “Slug Inventory” in worksheet 1. The “Slug Usage” worksheet will be an ongoing list with daily entries. I need Excel to first find the matching Slug# in worksheet 1 (Slug Inventory), and then each time a matching slug number is entered into the (ongoing) worksheet, it will subtract from the inventory level in Worksheet 1.

2) Finding last match in a column: Worksheet 4, “Cell Locos” – team members need to view this table so that they know where to find the slug inventory. This table needs to pull from worksheet 2 (Inventory Rec.). When Shipping enters the “inventory received” into the Google form, they will also assign a cell location in which they are storing the slug inventory. I am using Index & Match functions, but of course it is only picking up the first matching value for the cell location ID, and I need it to pull the last matching value, as this worksheet will also be an ongoing worksheet with multiple entries (i.e. location cell CI-1 will have different slug inventory going in and out of it).


I just joined the Forum yesterday as I feel it will be a significant help to me. I hope this all makes sense. If you need further explanation, please let me know. In advance, thank you!

-Michelle