In the first place I dont do it that often in private projects because the estimation is always wrong.
At work i just think about best and worst case scenario and the average time it could take. If the the worst case scenario is really time intensive and there are a lot of factors that could go wrong in contrast to the best case, I significantly increase the estimated time for the task. Otherwise its 1/6 best case; 1/6 worst case; 4/6 average time