The dates are intended to define the computation period for the earnings calculation. In this case, the beginning of the computation period is the date that you made the first contribution for 2020.
The rate of growth has no bearing in the the amount that you recharacterize. The rate of growth only affects the calculation of the amount to be transferred. The amount transferred is the amount recharacterized, $2,400, plus the attributable earnings. You can't recharacterize more than the $2,400 contributed, nor is there any need to do so. Don't confuse the amount to be that must be transferred with the amount being recharacterized.
Example 2 of CFR 1.408-11 describes how to calculate the gain when there have been multiple contributions:
https://www.law.cornell.edu/cfr/text/26/1.408-11
I your case, the Adjusted Opening Balance is $2,400 plus the balance immediately prior to the first contribution made for 2020. The Adjusted Closing Balance is the balance immediately prior to the recharacterization.
