There is no set formula for putting a dollar value on the financial impact of a key employee's death. Nevertheless, you need to come up with a figure as a guide to how much insurance coverage to buy. Some life insurance companies provide formulas for this which may or may not have a realistic relationship to the employee's worth to your business.
In some cases, a look at the employee's responsibilities can facilitate valuation. If, for example, the employee is responsible for a certain volume of sales, the loss is the profit derived from the person's sales, less the profit that could be expected from a replacement.
Also to be included is the expected cost of replacing the employee, including employment agency fees and moving expenses and possibly a higher salary for the replacement.