@PS56k
For purposes of calculating the RMD, the IRS says,
More than one IRA.
If you have more than one traditional IRA, you must determine a separate required minimum distribution for each IRA. However, you can total these minimum amounts and take the total from any one or more of the IRAs.
My own preference would be to rollover the purely pre-tax 403(b) to a separate IRA account from the mixed traditional IRA you already have. However, it's not wrong or disallowed to put everything in one IRA. You need to keep track of the non-deductible contributions via form 8606. The yearly ending balances and yearly growth doesn't need to be tracked for tax purposes. . The taxable amount of your withdrawals will be determined using the basis you have tracked on form 8606.
