Adjusting taxable basis with inclusive tax item price

Our items are including Dutch VAT. If we want to sell the item to a foreign individual we need to change the taxable basis  (i.e. item price minus Dutch VAT) before calculating the foreign VAT.

So the formula should give us the outcome (itemprice / 1+Dutch VAT) * Foreign VAT

We are not able to change the taxable basis in a tax formula to get the correct outcome.

Base rate modifier is changing the rate (as the name suggests) so is not modifying the taxable basis.

Anyone an idea how to deal with this?

Thx

Rob Struijk