When modeling portfolio withdrawals, we use the following method:

1. If RMDs are required, these are always taken
2. If an account does not allow withdrawals (like IRAs before 59.5), we don’t take withdrawals from this account in the years when the age restriction applies
3. Otherwise, withdrawals are taken proportionally from all accounts. So, for example, if a $1.5 million portfolio consists of a $1 million account A and a $500k account B, 2/3 of the withdrawal will come from account A and 1/3 from account B. 

Important note - the ordering of your withdrawals will also be determined by your selected tax strategy.