percentage of the contribution would go to my tax return? Is it based on my highest marginal rate, since I would be reducing from the top end of my taxable income, or is it based on the effective tax rate?

If my wife and I both maxed out traditional IRA contributions ($5,500 each), and my effective tax rate is 15%, would I increase my tax return by $1,650 (15% of the $11,000), or would I increase my tax return by whatever that percentage is multiplied by $11,000?

Am I missing something entirely (in other words, is it more complicated than simply increasing my tax return by multiplying the IRA contribution by the effective or highest marginal tax rate)?

(I already filed a return and received my refund, but I would file an amended return.)