In a "backdoor" Roth conversion, taxpayers roll a nondeductible IRA into a Roth IRA. Because they already paid taxes on that amount, converting it to a Roth is expected to be be tax free (unless any earnings are also converted).
The problem is the pro-rata rule. The IRS doesn't let you just convert that "after-tax" money if you also have "pre-tax" money (like earnings or deductible contributions) in any of your Traditional IRAs. Instead, they treat all your IRAs as one big bucket and tax the conversion proportionally. For example, if 10% of your total IRA value is after-tax money, then only 10% of your conversion is tax-free, and the other 90% is taxed as ordinary income.
This is why TurboTax needs your total account value, it has to calculate that percentage. If you enter $1, the software thinks you have no other IRA money and might incorrectly compute the conversion.
Here is some information you may find helpful: Rollovers of after-tax contributions in retirement plans
