Procure a Kodak or equivalent "gray card". This is a battleship gray placard with a surface that reflects 18% of the ambient light. You can test such a surface for accuracy. Perhaps a local photofinisher will help. Most such shops routinely use an instrument called a densitometer. Such an instrument reads film by transmitted light and prints by reflected light. The gray card should read 0.75 density units.

Load black & white film in your camera. Use the spot meter to measure the gray card. The meters output will be the shutter speed and aperture. Set the meter to the box ISO speed of the film. Shoot a series of exposures of the gray card. I suggest you use 1/3 f-stop increments. Also, shoot close-ups of the gray card i.e. the entire frame is an image of the gray card. Develop the film per specifications of the developer used.

Have the shop measure each frame plus the a reading from a clear film area (D-minimum). The frame that reads 0.75 + the clear film reading will be the correct exposure.

Assume the clear film (D-minimum) reads 0.12, then the frame with the correct exposure will read 0.75 + 0.12 = 0.82.

Lots of pitfalls when calibrating a meter.

0.05 = 1/6 f-stop
0.10 = 1/3 f-stop
0.15 = 1/2 f-stop
0.20 = 2/3 f-stop
0.30 = 1 f-stop