My #1 rule for guesstimating is take your greatest possible estimate, double that and then add 20%. It will still probably take longer, but probably not too much 😄. This came from years of complete failures on my part, to get even close to a correct estimate for anything 🙄.
My thing is there's always something in the process that i forget about when i make the estimate, like beta testing, or play testing on my end, or claire's voice work, etc. That combined with the randomness of life. If I could just sit and work on the game all day, every day, that'd be ideal. But sadly that's not the case XD