sorry 20% was a typo

Corrected version:
Set Configuration > My Store > Display Prices with Tax > true
Set your tax rate/tax class to 19%
Set your currency to EUR (with decimal places = 2)
This is the configuration for almost all German shops as the German VAT is 19% (or for some products 7%)
Create a product with the following price:
Products Price (Gross): 0.90
Tax Class: 19%
The net price will be: 0.7563