After skipping several updates(i altered the code to much) i installed the last version(1.4.9)
I noticed that the item totals are wrong calculated when adding discount.
The tax amount is not correct.
Discussed it already a long time ago in this post: bug
The problem is that this can not be changed easily without breaking any existing records. The only way to solve this would be to add an option that allows the user to set the order in which the amounts are calculated.
Unfortunately the option is not working correctly. See this thread for more information: InvoicePlane needs your help testing a new feature: calculation order
It would be possible for fresh installations but we simply cannot tell all existing users to start again. And the problem cannot be solved with an update for existing users.