I use parameters and spreadsheet formulas to model the behaviour of different items in the budget over time. Some items are incremented by an absolute amount or a growth percentage every month while in other cases I want this percentage to be gradually decreasing as time goes by. In some cases a budget item is a function of another item in the plan, such as overheads per headcount or infrastructure costs per customer served.
The most useful part of a financial plan is the parameters and models used to calculate the time series for each line. This is where we can see the business logic and underlying assumptions that govern the entire budget. It’s the difference between a plan and a bunch of numbers.