I usually do 2x my expectation. I keep track of my prior estimates and compare against my real time. I normally miss by about 30% - 50%, this leaves me a fair m

I usually do 2x my expectation. I keep track of my prior estimates and compare against my real time. I normally miss by about 30% - 50%, this leaves me a fair margin. It’s also been improving with time, which means I’m getting better at assessing my skills and problems. For me, I tend to work quicker than other contractors / employees, so I still often beat out the other quotes (time or money). That’s not always the case for others, so they margin of safety may not be there. In which case, multiply by a smaller constant.