I don't have a Yashica, Patrick, but if your controlled tests show it to be accurate, I'd think you'd be justified in relying on it. The trick to any meter, however, is to understand what it is measuring, and then make any appropriate adjustments based on the scene and the objective for the image.
You might, for example, try to determine the field of view of the meter, and mark that on the viewing screen. Then, when metering a scene, you can more accurately assess whether adjustments are required. If, for example, there are large areas of white in the area being metered, you can bet the negative would be correspondingly underexposed at the suggested exposure.