The Flexible Retirement Planner is a Monte Carlo Simulation calculator. This lets you find ways to improve your retirement plan. The calculator uses your inputs to run 10,000 market simulations to determine your retirement's probability of success.
The planner has an additional inputs section. Here you can factor in expenses such as college costs for your children. It also allows you to include any additional income that may come in only occasionally. With this, you can set the specific years the income may come in to get a more accurate result.
Additionally, you can set portfolio return for specific years. That way if you plan to be more aggressive with your investments when younger but less aggressive as you get older, you can again plan more accurately.