Tax Base Amount (tax on purchases)

[Perlita]

I need to define the Tax Base Amount of “one” Tax Code with two different “processing keys” (for example: VST, NVT) with the following request:
* The first “processing key” must apply the percentage only if the base “Tax Base Amonunt” is less than a “maximum amount” established.
* The other “processing key” must apply the percentage only if the “Tax Base Amount” is higher than a “minimum amount” established. In this case the percentage must apply to a Base that is “Tax Base Amount – Minimum amount”
The problem is that I need to use "only one tax code" assigned to the expense in the invoice, I cannot use two tax codes.
Is there any exit program to put a table and a program in the moment that SAP builds the Tax Base and apply the Tax Code?
Thanks