Because this kind of planning is dealing with an uncertain future, it requires making REASONABLE estimates in this forward projecting, and then redoing the projection each year, adjusting savings rate based on how variables actually performed. This is an actuarial process, involving the ability to perform time value of money calculations based on cash flow projections and is quite similar to determing funding levels for defined benefit pension plans.

Each person will be different, due to assumptions and income assets they have available to them (such as a pension), as well as lifestyle expectations at retirement. General formulas or charts, in my experience, can be close for some and a mile off for others.

Retirement savings need is a 3-step actuarial calculation, that once understood, really isn't all that difficult. Not sure why these web sites that purportedly speak to retirement, don't take the time to explain this and build a savings calculator that shows each step in leading to the final savings number.

BruceM