I would say that the threshold should be closer to 30k. As Coolidge said, there taxes should be fewer, and less people paying them.
So how much would that flat tax rate be?
I think that would need to be determined by people smarter than us. Get a bunch of brilliant economists & mathematicians together to iron out the options then put a few plans on the desks of Congress. Let people who know mathematics do the numbers rather than politicians who's mathematical knowledge may not even surpass mine.
Not really, the general gist is that flat tax with set deduction is a type of more angular progressive tax.
It is steeper at onset and flatter on top, so rich pay less while middle class pays more:
Note that this is $25K deduction and requires around 35% rate, a 30k deduction would be more like 40% rate.
Note also that this discussion is about simplifying the tax code to eliminate deductions, which does not require a flat tax approach. We could keep the same system we have now, and just eliminate deductions. Or we could tweak the current system to reflect the changes brought on by the elimination of deductions.
Ultimately, the elimination of deductions is a pretty good avenue to go, in whatever form. The simple fact of the matter is that the monstrosity that is our tax code is primarily designed to cater to political interests more than anything else. Deductions for this, deductions for that. I wouldn't be surprised if there isn't a deduction for manually inseminating goats for human hybridization. Eliminating the deduction based approach to our tax code is an important step to creating transparency.