No more than the amount contributed during the taxable year, plus its gain should be transferred. However, if more is transferred, the broker would probably just report the additional amount as part of the gain. Transferring more than required is not to your benefit. If the broker instead reports on the code R 2018 Form 1099-R an amount in box 1 minus the amount in box 2a that implies that you recharacterized more than you contributed to the Roth IRA, the IRS could take issue with it, if they detect the discrepancy at all. (If you have a loss and the amount in box 1 of the From 1099-R is therefore less than the amount recharacterized, the IRA only knows from your explanation statement the amount recharacterized since box 2a will have a zero, not a negative number.)
There is probably no need to wait to submit your tax return since your explanation should agree with the amount you told the broker was the amount being recharacterized and the amount that you told the broker was the gain. The only reason you might wait is to ensure that the broker follows your instructions so that you know that your explanation statement agrees with what the broker does. But you won't receive the code R 2018 Form 1099-R until next year. You'll have to confirm that the transfer took place by reviewing any transaction statements or account statements.
