Take a guess. Double that and add 10%. Then with any luck at all you'll only be under by 50%. <grin/>
Seriously, if it's a one-person project, use your best estimate of your own skills. I wouldn't show the detailed estimate if I could help it. After all, something *always* goes wrong in one aspect of a project, and so you hope/expect to make it up in other aspects.
An optimist sees the glass as half full.
A pessimist sees the glass as half empty.
A realist drinks it no matter how much there is.