How do I calculate Roth tax saving at retirement.
Suppose
Input summary
Annual retirement plan contribution: $3,600
Years to invest until retirement plan withdrawals begin: 20
Hypothetical annual return before retirement: 8%
Current federal income tax rate: 25%
Number of annual withdrawals: 20
Hypothetical annual return in retirement: 6%
Estimated federal income tax rate in retirement: 25%
Traditional and Roth accounts: Equal value at retirement
After 20($InvestmentPeriod) years of contributions and growth, the traditional and Roth accounts would both be worth $177884($TotalContribution) at retirement. Without accounting for taxes, each account could provide annual withdrawals of $14807 in retirement.

How do u I calculate these values? $177884 and $14807

Are you asking how whoever did this analysis came up with the results? They seem to have simply done a cash flow using the data you provided. The account would indeed grow to $177884 after 20 years of contributions. You can then take withdrawals of $14807 per year, but you woud run out of money after 20 years - this doesn't sound like a very good retirement plan to me.