I have a line item Headcount and my current year is FY17.
For FY18 I need headcount value to be prior year headcount and FY19 to be FY18+FY17 headcount value.
Can I please get some guidance.
You'll need two line items - one to enter/hold the headcount change, and one with a formula to maintain a running balance over time - use the PREVIOUS function to access the last value and add the change. You might also want to look into opening/closing balances.