Need help fixing code that causes one cell to change based on the entry in another ce

Parts of the following code work, parts don't. Can someone evaluate this and let me know how to fix it?

Here's the breakdown:

I want to enter a specific quantity in a cell and have a corresponding cell change based on the number entered in the first cell.

SPECIFIC EXAMPLE:
I enter a number <=3030 in Cell D1 and it automatically changes cell B1 from "43" to "408". Also, if cell D1 is empty, cell B1 defaults to "43".
I enter a number <=8500 in cell D2 and it automatically changes cell B2 from "398" to "406". Also, if cell D2 is empty, cell B2 defaults to "398".

I had help with this in another thread, but it started getting a bit convoluted.

I'm very new to this and with help (and my own "logic") came up with the following code (I know it's broken and not ordered right, but I don't have a lot of knowledge with VBA but am trying to learn so go easy on me!! ):

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BRIEF OVERVIEW:

This is a customizable billing sheet. There are 2 columns (B and N) where certain item numbers (1 - 999) will be entered for customization purposes and then saved for each individual customer. The item numbers in those columns may be changed at any point depending on the customers needs. "43", "398", "406" and "408" are just 4 examples of some of the item numbers. Columns D and P contain the cells where the quantities for each item are entered. here's a screenshot:

http://img.photobucket.com/albums/v6...EENCAPTURE.jpg

Joshiah, can you lay out a table that shows your logic? It would be easier to code from something like this:

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Josiah

As I have mentioned on MrExcel, your code has logical conflict.
See MrExcel.