The Savings account uses the same rate of return for the combined Savings, Checking and Brokerage, when in reality this will be separate.

The reality is you might be getting 1% compound interest in your Savings and Checking account, and 7% rate of return on your Brokerage account. However you might have 90% of your after-tax assets in the Brokerage account, earning the best return. The gains on your Savings and Checking are small. Since there is a need to have a single Savings account within the model, there is a single rate of return to set.