The taxable amount does not depend in any way on which traditional IRA the distribution came from. All of your traditional IRA accounts are considered together to be a single IRA with the taxable amount being calculated using the sum of the year end values in all of your traditional IRAs. There is no such thing as a "deductible IRA," only nondeductible contributions made to your traditional IRAs in aggregate.
The calculation is done separately for your distributions and your spouse's distributions.
