to repeat what I've looked on this site a month ago, taxes are as followed :
for FO's :
gross salary = 56500
- 8% of National Insurance tax to the UK
- 25% of income tax to Portugal
=> 56500 - 8% ~ 52000 €
52000 - 25% ~ 39000 € ~ 3250 € / month net salary
Then you can add extra days if you comply with (~250€/month) and per diem. But per diem are used to pay food, transportation to your gateway, ...etc.
For CPT, do the same calculation and you'll find ~ 5400€/month without per diem and extra days.
I suggest you contact the income tax department of your country to check if there is any agreement between Portugal and your country. In Europe, some countries have it and it is not necessary to pay other taxes in our own country. Northern countries don't seem to have these agreement
fred