[ppeetteerr]

If this is the first time you estimate a set of work, you will not have the experience to provide an accurate value. You may guesstimate, but then you'll have to multiply that value 2-4x to make sure you don't under-bill. Now, that multiplier is key, it's your buffer.

If you don't have a multiplier, expect to work overtime or divide your revenue.

As you get more comfortable with your estimates and your tools, you can reduce the value of that multiple. I've been able to get it down to 1.5x but the scope of the work was limited and our team was very familiar with the code.

As for the proposal document, there may be templates online. Some agencies create an exact list of features, and wireframes so that there is no scope creep. I recommend you do the same if you have time (some agencies with a good reputation bill for this work). Also, allow your client only two small changes to the design. Anything more, and it's extra money.

Your main problem with quoting will always be properly estimating the amount of work it takes to complete a project. The same project, with two different clients, may go from profitable to a loss. It's very important to pad the work and to gain a lot of experience in a small number of tools so that you may get better at estimating. Specialization is key.

You'll hear some people say that they bill iteratively. It's possible but not every client wants to hear that their project has an undefined cost. You'll still have to cap it at some value in which you can deliver the work.