To register new parts:

On Parts sheet, either:
1. Activate bottom right cell of Table tParts, and then press Tab to add a new row to table. Into new row enter part number and quantity to keep for this part;
2. Activate any cell in Table tParts. From Menu select Design, and then at left of menu pane click on <Resize Table button>. In Resize Table popup window set new table range (edit the row number, and press <OK>;
3. Simply enter new part number into next row immediately below last row of Table.

Quantity to order is calculated automatically in table. NB! In my table is error in formula for QtyToOrder! The right formula is
You enter the correct formula into any cell in Table field, and it is corrected for whole field.

When you enter the QtyOnHand after inventory into Table tInventory, the quantity needed to increase current quantity to QtyToKeep is calculated in field QtyToOrder. When you purchase this part and enter purchased quantity into field QtyPurchased, the QtyToOrder is decreased accordingly (but never gets negative).

All formulas/data validations/formats are copied automatically into new rows of tables, so long as they remain same for whole column.

On PartOrder sheet you select inventory date, which serves as ID for inventory, and all parts with QtyToOrder > 0 for this inventory are listed in table. You have to decide yourself, how much different parts you'll have, and set the number of Table tPartOrder accordingly (my advice is, multiply the number of parts you think you need with 2 or 3). Btw, the easiest way to expand the table is select all table sells in last row, and drag them down.

The inventory sheet can hold many inventories (you need a new table or to delete old inventories after the number of rows grows over 20000 or even more). Before enter new inventory data into tInventory (there is spelling error in my file, correct tInventoty to tInventory), you have to register the new inventory date in Table tInventories on sheet Inventories.