Everything I read shows the 23% / 30% figure, depending on how its calculated. You will never convince me that 5% will cover it. Our federal gov't is far larger than 5% of GDP. I wish 5% would cover it but it won't, without drastic cuts in gov't size and a huge economic boom. Of course, I wouldn't mind if both of those happened.
Actually, I thought that we spent about 5% of GDP on military alone.
Actually nobody knows what percentage will work.
But income tax percentages and sales tax percentages don't relate to one another.
In anycase try the lower end first, and we can see after the 1st year, it can work.