Client converted $24,000 of Trad IRA to Roth IRA in early 2018, her Trad IRA had a balance of $80,000 as of 12/31/2018. $37,500 of Trad IRA were non deductible IRA contributions. To determine basis for 2018 Roth conversion amount of $24,000, my assumption are as follows;

1) Take Trad IRA FMV of $80,000(as of 12/31/18)
2) Add back Roth conversion amount($24,000)
3) Divide Non Deductible contribution amount of $37,500 by $104,000 to get pro rata % of 36%
4) Of the $24,000 converted, $15,360 is taxable and $8,640 is basis

Is my methodology and math accurate based on facts submitted?