The amount that you are recharacterizing is the amount you originally contributed. The amount to be transferred is the amount being recharacterized plus the gains attributable to the amount being recharacterized.
In this case the date of the contribution/conversion is the date on which the contribution being recharacterized was made to the Roth IRA. The earnings calculation must be done on the Roth IRA account as a whole. Assuming no other contributions to or distributions from this Roth IRA account between the date contribution being recharacterized was made to the Roth IRA and the date of the recharacterization, the amount to be transferred to the traditional IRA will be:
Roth contribution * account FMV immediately prior to recharacterization / account FMV immediately after contribution.
See CFR 1.408-11 for the calculation details: https://www.law.cornell.edu/cfr/text/26/1.408-11
