Thanks, but I think I'm lacking clarity because I can't use actual figures. Let's try this:
A=Total Cost Basis as reported on final 8606
B=Aggregate IRA funds subject to RMD on DEC 31, 2018
C=Aggregate IRA funds subject to RMD on DEC 31, 2019 (required by 2019 TT)
X=RMD taken this year upon age 70 1/2
MY calculation for nontaxable 2019 distributions derived: (A/B)*X
TT Premier derived calculation from 2019 8606: (A/(C+X))*X
Note that it not only uses a (significantly higher) 2019 Aggregate (vs 2018), but additionally it ADDS the RMD to that basis in the denominator.
These numbers might be used for future proration, but the difference I've cited on 2019 8606 result in an additional several hundred dollars in taxable income this year's filing.
The first year really shouldn't be this difficult, should it? I understand in the future the cost basis will be reduced by the non-taxable amount used this and subsequent years but I have provided for that in my spreadsheet calculations.
