Ray made a great recommendation. Here is the W-4 tool from IRS.gov. Not sure if you saw the YouTube video with instructions on using the tool. Click here to watch. I would go back through tool after reviewing the video. It should answer your questions on the negative amounts.
As a rule of thumb, the spouse with the higher income should take fewer exemptions on their W-4. Remember, on your tax return you are combining incomes that will push you up into higher tax rates. Having the higher earner withhold more tax will offset some of the additional tax from combining both your incomes.
