How much estimated tax do I owe on the taxable part of my fellowships?

You must take into account your husband's income when you calculate your gross income. That, along with your earnings, your taxable fellowships, and any income from interest or dividends, etc., make up your gross income. You need to take a look at how much is being withheld from your pay for income tax purposes to see if that is enough to cover what you would owe at the end of the year, taking into account the standard deduction and the personal exemptions.