Wow, I don't think I have had so many comments, thanks guys for leaving your thoughts, it seems like something people care about......am so glad some of you are going to do it along with me....
OK, here is a link the the spread sheet I made with the help of Mr Wool (Who trained as an accountant and he is super expert on this)
It has an example working in it.
There will inevitably be things which need to be changed to suit each person/business, if you have any questions pop them in the comments, and we'll see what we can do....save a copy and have a play.......I would urge you to also estimate costs of a show if you are thinking about doing one, and then figure out how much you would have to take to at least break even....then calculate how many units you would have to sell to make that happen, it might shock you!
Let me know how you get on if you try it...what will differ for everyone is the cost of their materials, for most shops it will be 50% of the takings, if you make the product you are selling it will be different.