So I am not directly answering tips for estimating, but I implore you to not use flat fee or value based unless you have an amazing grasp on both the project an

So I am not directly answering tips for estimating, but I implore you to not use flat fee or value based unless you have an amazing grasp on both the project and client AND you are prepared to have an attorney help you put together a very very detailed agreement. Here is the problem, flat fee projects and value based projects can be great, if you have a good client and you can reasonably predict the project scope and have a reasonable estimation of the possible unknowns. I have worked this way quite a number of times over many years in consulting. What I found is that most times, even with a decent client and a good estimation, a flat fee project almost always looses money. A value based contract on the other hand doesn't generally loose money but it can be hard to estimate and hard to sell to a client depending on the contract value and your experience. So, unless you have an excellent handle on the project and have an excellent handle on the client, never do a flat free project. A value based project has it's own headaches and should have significant contractual obligations on both sides to insure neither party is injured because of the other. For a consultant, this means essentially it has to state, I am building X (in superb fine grained detail) and client is paying Y at each milestone (in again the same level of detail), and any variance or deviation from either will result in additional fees. In fairness to the client, it should also state that you don't get paid for any milestone until you deliver that milestone, each milestone should contain a specific testing procedure and delivery procedure to validate it was completed and delivered. If you are late delivering, the extra time is on you. This is a bit of an oversimplification, but it hopefully gets the point across. IMO, unless you have an excellent grasp avoid both flat free and value projects and stick to either Hourly or Weekly billing. I prefer a weekly billing method because it forces clients to spend in larger chunks which helps you control the situation a little better (not perfect but better). Not to mention, you save yourself some paperwork and time in how you invoice and manage your time. Hourly is still very viable, and while it comes with extra headaches in managing and invoicing time properly, it is the safest generally for a newer consultant.