Representing remaining savings to be made, i.e. savings up to now are disregarded. A typical value could be annual spending divided by annual return on investment (roi) minus current savings.
In the next year
Relative to previous year, assumed to be constant for all coming years
Under the assumption that all savings are invested and that this roi is identical across years
Under the assumption that this doesn't change over the years.
Please choose distinct values for annual savings increase and return on investment.