Some additional details related to your original question:
AGI is not a factor in determining your permissible SEP-IRA contribution or self-employed retirement deduction. However, a permissible SEP-IRA contribution reduces your MAGI for the purpose of being able to deduct a regular traditional IRA contribution, but it also means that you are contributing to a workplace retirement plan which makes this MAGI relevant if it was not already so due to participating in some other employer's plan. As you found from the definition of MAGI for the purpose of being able to deduct a traditional IRA contribution, MAGI is determined without any potential deduction for a traditional IRA contribution, so there is no circular definition of this MAGI.
