Tejomaya Technologies has 1,000 staff, a mid-year hike, 18% attrition backfilled after two months, and 150 net new hires. Build the year's staff cost budget month by month.

The year costs about Rs 169.3 crore, not the Rs 189.8 crore of the first draft. Build it monthly: the original roster shrinks by 15 heads a month, backfills arrive two months later at 95%, new hires add 12.5 a month, and the rate steps from Rs 15 lakh to Rs 16.5 lakh in July. Monthly cost runs from Rs 12.47 crore in April to Rs 15.30 crore in March. The bridge from the Rs 150 crore base is the hike, less vacancy savings, plus new hires, each a timing question.

A household that hires a cook in October does not budget twelve months of her salary; a family whose rent rises in July pays the old rent for three months. Staff cost is headcount times rate, month by month, and both move during the year, so the annual number is the sum of twelve different months. Tejomaya has three moving parts: the rate steps up in July, leavers thin the original roster every month, and two kinds of joiners add to it on their own schedules. Multiply year-end heads by the year-end rate and all three timing effects are lost.

Three head counts, one rate. The original roster loses 15 people on the first of each month, so in month m it has 1,000 less 15m. Backfills for those leavers arrive two months later at 95% of cost, so in month m there are 15 times (m minus 2) of them. New hires add 12.5 a month, so 12.5m. Each count is multiplied by the going rate over twelve: Rs 15 lakh until June and Rs 16.5 lakh from July, with backfills at 95% of it. April is 985 originals and 12.5 new hires at Rs 15 lakh, Rs 12.47 crore; March is 820 originals, 150 backfills and 150 new hires at the hiked rate, Rs 15.30 crore.

Start at 1,000 heads for a year at Rs 15 lakh, Rs 150 crore. The hike adds Rs 11.25 crore, nine months of 10% on the base; vacancies save Rs 5.22 crore, because each of 180 leavers leaves a seat empty for two months and the replacement costs 5% less; and the new hires add Rs 13.31 crore, which is far less than 150 times Rs 16.5 lakh because the average new hire is on the rolls for only half the year. That sums to Rs 169.3 crore. The draft's Rs 189.8 crore applied the year-end headcount and the year-end rate to all twelve months, which overstates by about Rs 20 crore, enough to misprice every project the centre bills.

Say what the model does not know. Attrition is rarely even; it spikes after bonuses and hikes, which pulls leavers forward and raises the vacancy saving while hurting delivery. Backfill at two months is an average that hides long searches for senior roles. And joiners' cost to company is an assumption until the offers are signed. The finance lead should hold the roll-forward and reforecast it quarterly against actual joiners and leavers, not defend one annual figure.