Help With Calculating Percentages & relating numeric values to letters

Im trying to create a Purchase order form, amongst other problems I'm facing, the one I'm trying to tackle at the moment is as follows,

I'm trying to use letters as 'code' for Tax percentages, take a look at the following

Screen Shot 2016-08-30 at 21.13.15.png

so after i have entered the quantity and the unit price, the 'amount' column will populate with a simple
'=[unit price]*[quantity]' when i then enter 'S' into the 'TAX' Column i want it to add 20% onto the 'amount' column

but just to make it that bit trickier i have about three different letters representing different tax percentages

I'm pretty much a noob to excel, I'm pretty literate and capable but simple explanations would help haha

many thanks

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after that.

1. It would help to see the workbook.

You get better help on your question if you add a small excel file, without confidential information.

Please also add manualy the expected result in your file.

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Hi, I do sincerely apologise and i will make sure in future i do not make that mistake again, if there is a way i can change the thread title i will do so. I'm fairly new to this!

I have attached a copy of the spreadsheet, it has some company info in it but i do not feel it is sensitive as its readily available in the public domain anyway.

Maybe it will make it easier for people to understand what I'm trying to do

eventually i want to make this so i have
the total VAT
sequential P.O Numbers that automatically pick up where the last one left off
a button that will automatically save the PO as a pdf

To change a Title go to your first post, click EDIT then Go Advanced and change your title, if 2 days have passed ask a moderator to do it for you.

I think i can be done, if you edit the first question. (the edit button is on the right side hand).

What are the letters and their associated tax rates?

Good job !