Subject x centered on the paper and a camera position x at the margin. Draw a line through both clear across the paper to the background.
At the subject's position note the camera setting you are going to use. This reading, IMO, is best taken with the back of the meter touching the subjects nose, the dome extended, and pointed directly at the camera. This reading should always match the planned setting. (Studio lighting is adjusted to the camera's need.)
Now for the back ground do you want it darker or lighter and by how much. So write on the map + or - 1, 2, ... Or whatever. Adjust the lighting there.
For right to left differences, measure with dome retracted, meter pointed at each light source.
The ratios, right to left, that you want are purely subjective. If you keep good notes the need for digital is minimal. The only thing digital is, is faster.
I find mapping is the best way for me to understand what's happening, analog or digital.
With a Polaroid (by whatever means) I am looking for things I've missed; like trash in the scene.