Comfort Zone

We've all heard the saying;" push yourself out of your comfort zone," before and for various reasons. The most obvious reason in my opinion is to obtain personal growth through exposure to new or unfamiliar things, but how does one know where the boundaries, of just how far you are willing to push yourself, lie when your "comfort-ability" with your new endeavor can no longer serve as a guide? There's,after all, truth to the statement that too much of anything is a bad thing, thus venturing too far out of your comfort zone may have negative results.
When all other guides fail I believe we should evaluate what we hope to achieve (through daring to try out new things) and let our ultimate expectations help in choosing just how far we willing to go. Life is a series of calculated risks and we'll never know just what the rewards may be if we never challenge ourselves.