I did add a new product but realised that the tax price in the add product page did not update.
So I searched the documentation and did set up the tax.
afther going back to the product I still could not change the price, the price with and without the tax did keep the same. It did not calculate my newly added tax.
When I add a new product the tax updates fine witout any problem, but the product that was all ready created does not calculate the new tax, whenever I change the price.
I was lucky, I tried it out with one product first, and did not create many items first.
I guess this is a little bug?