Hi there. I think you've made a mistake in your calculations of what 8% tax would be. It looks like your Magento is configured for the prices that you enter into admin to be assumed that they already include tax, i.e. 539 is the inc tax price. To calculate the excl tax price (and hence the amount of tax that's been applied), Magento needs to calculate what excl tax price, when 8% is added on, comes to 539.
The maths to do this is:
539 / 1.08 = 499.074
The amount of tax applied is 539 - 499.074 which equals 39.93. The tax that magento correctly. shows.
To validate this, we can go the other way we consider what is 108% of the excl tax price
499.074 * 1.08 = 539.
This comes down to the fact that to calculate what 8% tax is, you can't take 92% of the inc tax price. As this is a calculate based on the excl tax + tax price. You instead have to reverse the exact process by which you added tax.
I hope that made some sense! I'm not sure I explained it very well!
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