The length of the frame, either width or length, is whatever the poster wants it to be. It is all dependent upon the width of the frame pieces and what makes for a correlated appearance.
The only thing that has to be calculated is the rabbit. That is what was present be Frank. The poster must decide the overall size the frame must be to look like he/she wants and then decide what the finished rabbit measurements will be. He/she must account for any matting besides the actual item to be framed.
Different types of media have different relationships of image to frame. For example, photos can have a mat border or just the white border on the photo. Watercolors usually have a mat border. Posters may or may not be matted. Prints may or may not be matted. Oil paintings have a stretcher frame behind the canvas. Etc, Etc.
The rabbet will be a different width/depth depending on the subject to be framed and how much mat or image exposure is desirable. This is not an arbitrary dimension, rather a matter of what is functionally appropriate. The frame stock width is the variable in the equation in my opinion. Frames come in all manner of widths and materials, so the relief from the rabbet will vary within a range, but it will not be arbitrarily 2" or some other amount.
The overall size of the frame will depend on the subject matter, whether there is a mat border, the amount of relief in the rabbet, and the width of the framing material.
To clarify my earlier formula:
2 X (molding width (-) rabbet) + photo length = OAL