If you try to create a breakpoint at which people will have earned enough (some amount $Y) that they will not receive the handout, you need to pay close attention to what kind of perverse incentive that has. $800 a month is $9.600 a year in non-taxed salary. Assuming that all payroll taxes still remain to fund the program (roughly 23% in Finland), and that you're in the 21% income bracket, that's equivalent to about $16k in salary a year. What you're going to find is that there will be a 'dead zone' in wages at the $Y to $Y+$16k range. In that space it's more profitable to take no raise and continue to draw your $800 monthly.
Of course, you can set $Y so high that the $800 monthly is small enough potatoes that no one really does this, but at that point the number of people is also a minuscule fraction of your population and the effort to cull them from automatic payments, even if mostly automated, will be more expensive to run than to pay them anyway. Programs for universal wages should be universal -- methods to exclude people cause weird and perverse incentives or just cost more to execute than not.
