You need to ask your custodian that question. You probably will not be able to do an exact $ amount.
You have an even more important question to ask your custodian: will you even be allowed to do a re-characterization?
Part of the new tax law eliminated the ability to re-characterize a conversion after 1-1-18. There's some debate whether that applies to 2017 conversions, but the majority of opinions say it does.
Please let us know what they say.
