Is Stock Income subject to higher tax rate?

[Marjie33]

When stock is vested, it is taxed at 20%.  As I'm in a higher tax bracket, I withhold an additional $800 per month to compensate.  Yet, every year I owe double digit Federal taxes.

I do what I can to effect my AGI (put money in 401k, put money in HSA, itemize deductions) yet don't seem to take enough out.  

What's the easiest way to figure out my tax liability and ensure I'm taking enough out of the paycheck so I don't owe a huge chunk end of year?

[DawnC]

Use the W-4 calculator in TurboTax.   That link has instructions.  If you are in a high tax bracket, the 20% withholding is not enough.  

Employee Stock