Retirement tax questions

This is my first year to take a distribution out of  the IRA.  My problem here is the Investment company rollover my 401K to Traditional IRA then they combine my post-tax IRA into one.

Here are my basic cost contribution to the (401K) IRA   $311298.00  (pre-tax)

                       basic cost contribution to IRA                        $65869.00 (post-tax)

                       current balance of the combine IRA           $900000.00

This 2022 take out (distribution) $30000 from the combine IRA

How do I know how much tax to pay for $30000 distribution? 

(65869/311298=21%) should I subtract

30000-6300(21%)= 23700 and paid tax on this IRA amount?

In Turbo Tax section Retirement Plans and Social Security

Select Update on IRA, 401K pension withdraws (1099-R)

Then update  line 2a Taxable amount of 1099-R form from 30000 to 23700

Please advise.

Thien