Earlier quoted context omitted.
As a dev who has quite some freedom managing himself, I learned to multiply my own estimates by about 2-2.5. If I had a bad case of managers, I would probably multiply by four.
There's a paper that gives an exact number: 1.7x. This is the average time it goes over estimates, for code and writing tasks. The other numbers seem better, but you should not go below 1.7.
electrical engineer + IT dev now here - literally every project I've taken part was 3x the estimate.