Yes, the calculations are easy, whether it's a flat tax or a 5 step progressive tax as Panda pointed out. What you are mentioning here is that the various tax loopholes are what cause the difficulty; you are correct also. Flat-tax people say they will get rid of the loopholes; but we could also get rid of the loopholes under a progressive system.
Here's what I personally see as the problem with a flat tax:
Let's set it at 20% to keep the math easy (it's early here in California).
A person making just under $10 an hour - they make about $20,000 a year. They will pay $4,000 in taxes. That leaves them with $16,000 for rent, food, utilities, transportation, clothing, etc. That $4,000 really hurts. It's not easy to survive on $16,000 a year.
Someone making $100,000 a year - they would pay $20,000 in taxes, leaving them $80,000 for that same rent, food, utilities, transportation, clothing, etc. They're going to be ok. Maybe they buy a Ford instead of a Mercedes, but they'll be comfortable and probably have some $$ left over for movies, etc.
Someone making $500,000 a year - they would pay $100,000 in taxes. That leaves them $400,000 for rent (well, probably mortgage), food, utilities, transportation (beemer!), clothing etc. Anyone saying they need more money? They may WANT more - we always want more- but they aren't close to the edge like the person making $20,000 a year.
That's why we have progressive taxes - let the $20,000/yr person keep all their money. Maybe $100,000 only gets to keep $75,000 instead of $80K. Or $70K. And that person making $500K? Maybe they can get by on $350K without starving much....