The purpose of the spot meter is to measure the contrast range of a scene. As mentioned before, depending on the type of film you are using you can best estimate the required cotrast range and exposure needed. For example, B&W film is capable of giving you printable information anywhere from 4 stops difference between dark and highlight to 8 stops or even 10 if you have done some testing and can adjust development. This is not the case with color film. With color film, be that negative or transparency you are limited by the development, which always has to be the same regardless of the contrast in your image.

For color film you can get away with about 5 o 6 stops contrast range, with transparency about 3 to 4 stops.

So the steps are, you take a measurement with your incamera spot meter of the dark part that you want to have detail, then you take a spot metering of the highlight, you see what is the difference in stops and use the metering from the dark part closing 2 stops from the recommended exposure (both aperture and shutter speed) Depending on the film you are using you will be able to determine whether you need devlopment controls or make a desicion as to have some elements go black without detail or go white without detail.