If you take the proposed eventual 800 Euro and assume every single citizen will get it (roughly 5.5 million in Finland) that would be 52.8 billion Euro/year. For comparison the net domestic product of Finland is 216.9 billion (bad indicator but one i could find with a quick search)
I think if you do this, every citizen should get the base income and only income above it and corporate income should be taxed. Currently the average income in Finland is about 3.1k/month. So on average that's 2300 Euro taxable income (once again grossly simplifying). You'd need to tax that at roughly 35% to refinance the base income. Not sure if the corporate taxes and hiking the 35% to say 50% (+savings on all the stuff that is now covered by the 800 like unemployment, retirement) can make up for all the other costs (infrastructure, education, health care, police, military etc.)
There's also the standard questions of "why not ask for more" and "what will people do with the free time, why should they work". Which are the interesting questions that an experiment could help answer.