How To Automatically Enter A Value Next In Sequence In A Cells

I was wondering how I could automatically enter a value next in a sequence in a cell when an adjacent cell has data in it and continue doing so indefinitely (or until the 65536th row). I could do this with autofil, but The sequence starts in the middle of the worksheet (there is a title and document data in the top few rows). The layout is as follows:

Cell A17 has "Part 1" in it (and always will before the form gets filled out), and all cells in colum A beneath that will be blank. Column B is where the user enters a dimension. So, at the start, entering a value into cell B17 won't do anything. However, if they enter a value into cell B18, I would like cell A18 to automatically be filled with the next part number in the sequence (in this case, "Part 2" in cell A18). The user will never skip rows when they enter data so the sequence will always be +1 to the previous cell in the column.

I'm doing this so they can just print out the completed sheet and not have to edit out the empty "Part *" cells that have no corrosponding dimensions.

Is there a simple way to tailor autofill to do this or would I need a macro?