Lets step away from your specific problem for a moment and get our minds around the concept.. (oooohhhhmmmmmmm) Ready?

You have, for example, two variables. For the excercise, lets call them A and B. Some input tells you which variable to use. Therein lies the problem, variables can't DIRECTLY reference variables.

Try this simple example, and I bet you will be able to solve your problem.
 Code:
Break On

$ = SetOption('NoVarsInStrings', 'On')

$A = 'Yes'
$B = 'No'

'Which variable would you like to see ("A" or "B")? '
Get $Answer
? ?

$ = Execute('$ActiveValue = $' + $Answer)

'The value of $' $Answer ' is "' $ActiveValue '"' ?

So, now expand this process into your problem. You define, say, 10 values, $A through $J. Your file says to use "E". You use the Execute code to assign the value of E to a working variable - $ActiveValue in this example.

I'll let you play on your own for a bit - it's not too hard to convert this to what you want. Stop back if you need more clues.

Glenn
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Actually I am a Rocket Scientist! \:D