The best resource I've found on estimating is "When Will it be Done?" by Vacanti. The great thing about the system outlined in the book is that it fits into any software development workflow system without requiring change and it provides probabilities along with the estimates.<p>It's not perfect though. It does require some historical knowledge of the cycle times of tickets. However, you could probably argue that without cycle time knowledge, estimation would be entirely fruitless anyway.<p>Having read several software estimation books, I can't say enough about this. It really should be the bible of software estimation.<p><a href="https://www.goodreads.com/book/show/40681093-when-will-it-be-done" rel="nofollow">https://www.goodreads.com/book/show/40681093-when-will-it-be...</a>