If u have 100k in gains

Only 50k is taxable

If your marginal rate is 20%

You owe $10k in taxes in total on the 100k gain  

 On 100% of the gain. 

$100 gain = $37 of tax

$100 gain * 50% = $50 taxable dollars * your income tax rate = amount of tax

Thus, if your tax rate is 45% then $22.5 of tax 

 Not on capital gains. I believe you’re thinking income.