Yes, there is a function in Excel for the inverse of the cumulative exponential distribution, which is EXPINV. This function can be used to calculate the safety factor for a desired service level.

To use the EXPINV function, you will need to know the mean of the exponential distribution, which represents the average time between events. You will also need to know the desired service level, which is typically expressed as a percentage.

Here are the steps to calculate the safety factor using the EXPINV function:

1. Determine the mean of the exponential distribution. This can be calculated using the
   Code:

   AVERAGE

   function if you have a set of data, or it can be provided to you if you are working with a specific distribution.
   
2. Determine the desired service level. This is typically expressed as a percentage, such as 95% or 99%.
   
3. Use the EXPINV function to calculate the safety factor. The syntax for the function is:
   
   
   Code:

   =EXPINV(probability, mean)

   where probability is the desired service level expressed as a decimal (e.g. 0.95 for 95%) and mean is the mean of the exponential distribution.
   
   For example, if the mean of the exponential distribution is 10 and the desired service level is 95%, the formula would be:
   
   
   Code:

   =EXPINV(0.95, 10)

   This would return the safety factor for the desired service level.