It is critical that a customer service group be invested in yearly, to maintain support levels as sales projections grow. To understand this quantitatively, try to find the average percent of net new sales that convert into new casework (case conversion rate). Then find the percentage of how much additional capacity you have with existing resources (existing capacity rate). Take the average of these two percentages, and increase your customer service team’s budget by that percentage of forecast sales. That amount is ‘maintaining’ the existing SLA in respect to the added case volume, and can be called the ‘maintenance rate’ of customer service investment.
A simple illustration: you expect net new sales to grow next year by $1m (25% year over year). After some spreadsheeting, you find that on average, 50% of new customers last year converted to support cases. You also know that you have capacity for only 25% more cases with existing resources. Therefore, the maintenance investment rate for your next year is 37.50%, or the average of your conversion rate and your existing capacity rate. And you now know that customer service will require an investment of ~$375,000 to maintain existing quality of service for the influx of new customers. You can test this formulaically: (Net new * case conversion %)-(Net new * existing capacity %).
In the real world, however, cost modelling is rarely this simple. What this formula assumes is that the rate of capacity is static (eg, no improved systems or training improvements) and that the rate of customer conversion is static with no trending (eg, 50% of new customers each year create new cases; that the rate hasn’t fluctuated). This formula also assumes that Sales will hit their net new target.
Therefore, a wise customer service manager models a budget as best she can, then proceeds to undercut that budget by a margin of error. This margin of error can be based on several things: the average percent of difference between forecasted and realized sales, forecasted and realized service case volume, or forecasted and realized capacity expectations.
And why under-, and not over-, invest for the coming year? Because it is less expensive, more agile, better for team morale, and your mental health, to employ more customer service agents or purchase additional resources due to shortage, then to have to dismiss redundant personnel or systems because you are over capacity.