Two points to elaborate, already mentioned in the neighbour thread on estimation.
Firstly, programmers are notoriously and famously bad at estimation, so it's highly likely that you do need to find the multiplier for yourself. This illustration is something like fifteen years old by now: https://pbs.twimg.com/media/EyNRiYQXMAIshEj.jpg
Secondly, wise managers know to expect this, and do the multiplication themselves. But if you have managers who do the opposite and always scale the estimation back, expecting the results earlier that promised, then you need to use a second multiplier—which the manager will unknowingly annul, getting closer back to your actual figure. Probably something around 1.5 to 2.
However, if you need the second multiplier then you also gotta ask yourself why you're staying in that company.