Retirement tax questions

Does the $20,000 part of the equation ever change in the formula for determining how much of the partial traditional IRA deduction I can claim?

 

Formula (for married file jointly, both have 401K plan at work):

 

Non Deductible part of Trad. IRA = ((MAGI / *116,000) / $20,000) x ***13,000 

*lower part of MAGI bracket provided by IRA