5700+1940 = * * * * ? (clue, not 9440 )
If you choose to pay the "non-resident" tax (24.75%
AFAIK) you will be taxed that percentage on everything I.E as FO 5100 + the block hrs @ 8€/hr.
So, assuming 80hr/mth you should receive net around 4300€.
As you are living in Spain, you would probably find it more lucrative to pay "normal" Spanish tax, which is around 24% (including social payments) , but only levied on the salary of 3800€.
I have a colleague who is living in Spain doing this, and he comes closer to achieving the figure you have extrapolated of 4828€.
For Capts, the 24.7% on everything, is possibly better, as a large percentage of the greater salary (7500) is taxed at a higher rate up around 40%, which nullifies the advantage of being taxed on salary only. Of course if you are properly integrated into the Spanish system & possibly receiving tax credits for wife/kids/mortgage/whatever you may still be better off as Capt paying "normal" tax, however none of the agencies seem sophisticated enough to arrange that for anyone, so you will be doing your own admin to benefit from any of that.
At the moment Contract Bases have 12 days off/month variable, so you will lose the stability of a fixed roster. 5/4 roster is on a 2mth "trial" at LGW base. . .we are hopeful (of course) it will become a permanent fixture/extended to other "Contract" bases.
Command "could" be quicker than 5 years with the planned expansion, depends on your total hrs obviously ( & whether you pass the Command course, our success rate is somewhat lower than many other companies . . . .strangely enough )