I use a combination of two things to estimate my time accurately: how long something will take that I have control over PLUS how long something will take that I have no control over. The latter sounds counterintuitive but I explain it
here.
I calculate how long something will take and then multiply by at least 2 depending on what it is. I add to this the amount of time that will be lost to "dark matter," i.e. stuff that is uncontrollable. In both cases, I use rules of thumb. I have never tried to create a fudge factor like Steve suggests. It sounds interesting so I may try it to see how well my rules of thumb have been doing.
Julie