The $68k to $78k range is a phase-out. Below $68,000, your entire contribution is deductible (making it the pre-tax contribution most people are used to with traditional accounts). The portion of your contribution that is deductible phases out at incomes between $68,000 and $78,000. At incomes above $78,000, none of the contribution is deductible.

I haven't looked at the phase-out schedule specific to this scenario, but when I looked in regards to the phase-out of the LLC (Lifetime Learning Credit), it was proportional. The phase-out occurs over $10,000 dollars, so 10% or $1,000 above the minimum threshold would make 10% of the contribution non-deductible.

Again, this is only if your employer offers a company-sponsored retirement plan. The numbers are different otherwise.