Excel has a limitation of 7 nested ifs - that is, an "if statement inside of another if statement".
You can see that your equation fails at the 8th if.
As you can see with your massive equation, it is absolutely NOT an efficient way of using Excel.
Komskies VLOOKUP is far more efficient. There are even better advanced techniques, but a VLOOKUP should be more than sufficient for what you need.
I made this lookup table based on your massive IF statement. It should work perfectly for you.
The left side you just enter those values in based on your pay scale (use the minimum of the range: ie. 0-28.57, I just put 0, and have 28.57 in the next row).