The methodology used in the loss projection can also be used to answer this question. The last exhibit in a Loss Forecaster loss projection shows you various pure loss rates. As the user, you will select an average of these rates in estimating your projected losses. However, each of these rates is an estimate of what the actual rate could be. Depending on the volatility of the loss experience, using the high and low pure loss rates as the endpoints of your range may be appropriate.
The endpoints should by no means be considered as absolutes. This is another situation where you may get a credentialed actuary involved, depending on how accurate of an estimate you are looking for.