Not easy to tell how much you could expect to earn as an FO but say the following happens.
Basic 3330 per month
40 sectors 440
130 hours duty 267
3 flexi work days 1074
Total: 5111 before tax
If you take BB's case, he has achieved 30 days flexi this year so about 3 on average, each month. I have achieved 17 so far and might push to 20. Some other FO I know has done 36 and one of our Captain would be in the 70 area.
But do not be mistaken, Flexi work is not the norm and certainly not at that level ! So when the operation settles down, none of this figure will be achieved by so many. Beware!
Flexi work is 40% taxable too and you should not forget that your pension will be taken off that amount as well as your share for the training cost (don't know how much because on different deal).
Cheers