GP does not mention the impact on its salary. Usually if you work 80% of your previous time, you are also payed 80% of the previous salary.
We have a progressive income tax here. The 20% comes from the part of my salary that gets the higher tax bracket. So in fact the decrease in amount I get paid is quite a bit less than 20%.
I also suggested this change at the yearly salary interview during which I negotiated a 10% raise. So really the amount I get paid each month didn't change all that much.