Render a few frames on your PC and get an average of them. That's the frame time you should put in the calculator.
The calculator works with animations only. To get the cost estimation for a still, do as follows:
First, render a test image with a 1/4 or 1/8 resolution. Then, multiply the render cost you get by 16 or 64 to get the full resolution cost. Note that these type of estimates aren't accurate, especially for higher resolution images.
This shouldn't happen. Try looking through the list again as some models might be placed in a different category (i.e. desktop, mobile, server etc). If your model isn't on the list, select the one nearest to it.
For Mac computers, check out this article that will help you find your CPU model "how to find your mac's exact cpu model"
The estimates generated in the calculator are only there to give you a rough idea of your scenes cost. However, you should render a few test frames beforehand to get a more accurate cost estimation.
The calculator shows farm render time in a scenario where either 100, 150 or 300 nodes are used in your render job, however, that number may vary. Also, it only shows the time of the nodes when they are rendering. It doesn't include the farm queue waiting time or the scene loading time.