These are five myths that still exist in retirement planning: Essentially, that means building an investment portfolio that leaves roughly 5 to 8% of your investable assets in cash. Once a month, you have a check for a predetermined amount electronically withdrawn from the cash account and deposited into your checking account. As the cash portion of your portfolio depletes to 3 to 5% of the value, you rebalance the portfolio and replenish the cash account.