There is no best formula to determine which account to withdraw funds from.
Withdrawals from your regular, taxable 401k and IRA's are determined by required minimum distributions (RMD's) so you have no choice on the minimum amount you must withdraw. If your RMD's plus SS income plus other income or savings in non-retirement accounts meet your needs you may want to take no withdrawals from the Roth account since Roth IRA's have no RMD and grow tax free.
You may even want to consider converting some of your taxable retirement account to a Roth. However, this will depend on your tax situation. If you have multiple accounts and significant assets it may be best to work out an initial plan the help of a CPA or CFA who can review all your finances and determine the most tax efficient approach.
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