The calculation is described in CFR 1.408-11: https://www.law.cornell.edu/cfr/text/26/1.408-11
I'll assume that the entire balance of the Roth IRA at Brokerage B was transferred to a previously empty Roth IRA at brokerage A and perform the calculation as if everything had stayed at Brokerage B since the transfer really didn't change anything with regard to the calculation. I'll also assume that the $7,000 contribution in January 2024 was for 2024 since it's too late to recharacterize a contribution made for 2023. I'll also assume that by April 15, 2025 you either filed your 2024 tax return or requested a filing extension for your 2024 tax return, otherwise the deadline to recharacterize a contribution made for 2024 has passed.
In your case the AOB is the value in the Roth IRA at Brokerage B in January 2024 immediately prior to contributing the first $7,000, plus the two $7,000 contributions. For the calculations that follow, I'll assume that the balance immediately prior to the January 2024 contribution was $12,000, but you'll need to redo the calculations using the true value.
The AOB is $12,000 + $7,000 + $7,000 = $26,000
If the recharacterization was done today, the ACB would be $26,604.
The Net Income attributable to the January 2024 $7,000 contribution would then be:
$7,000 * ($26,604 - $26,000) / $26,000 = $162.62 or 2.323% of $7,000
If your true balance prior to the first contribution was more than $12,604, you actually have a net loss.
