CL300,
Are you sure about the fact the tax applies on PerDiem in France ? It's quite surprising ! Does it mean that you pay 25% tax in Portugal and then you have to declare your Per Diem in France ?!
Don't know where you live, but I guess in that case that one could also declare all the professional expenses (Cleaning, food, etc...) and lower down the Per Diem declaration nearby 0 !!!
!!
French administration !!!