Understanding Long-Term Capital Gains Tax

Hello,

I am trying to understand the process of long-term capital gains tax (I want to contribute in my IRA to reach the 0% tax bracket for long-term capital gain).

Right now my taxable income is 95k (married filing jointly) and I have $8000 worth of long-term capital gain.

Currently, I should pay $1200 worth of long-term capital gains tax ($8000 @ 15%) within my total tax owed amount, which is around $8000.

For testing purposes, I reduce some income (bank interest) by $1000. My taxable income drops below 94k, which is under the threshold for tax free long-term capital gains.

However, my tax owed amount has only dropped by $400. Why ? If I don't pay tax on any long-term capital gains, I should save at least $1200.

Can someone explain this to me please? Is TurboTax incorrectly estimating my tax ?

Thank you.