Sadly the calculation issues still is there on some odd combinations.
Seems impossible to fix becaus of the way PW works. I'm starting to be quite sure it would be better if the tax percent was used to calculate the tax inside PW, insted of sending over a tax amount on each product. That would be the same way as a accounting system would work, and we're actually able to import the same order values (ex tax amount), and get it right in t.ex Mamut - but not in PW as of today.