You should have the 20% withheld when you transfer it. Or as the other answer said transfer the full 17,000 and send in an estimated payment to cover the tax. If you end up owing too much on your tax return there is an underpayment penalty.
Are you sending in quarterly estimates for your self employment income? You can increase one to cover the conversion amount. Just cross out the amount on the 1040ES and write in a new amount.
