Custom Tax calculation?

2 years ago
I do not mean the plugin but if I wan't to do a custom tax calculation when doing a sum of the cart etc.
Is it the same filter that I use to calculate the rest of cart/order?
2 years ago
What i mean is more about hijacking the actual vat-calculation so I can do my own custom. The actual rates I will use the plugin for. I just need to do a special calculation based on a custom "custom vat".

Where is the best place?

Thanks in advance.