Pardon my ignorance, my sensation is that the Gossen Luna-pro being a lightmeter with silicon blue sensor and integrated circuits, if it works, it should work just as factory new. Electronics don't "decay" with years. Selenium meters might lose sensitivity, I don't think SBC metres will show any kind of decay. So basically you are just checking for damage (needle out of calibration, "lens" displaced, who knows) that would put reading sensibly off.
Anyway, for testing any lightmeter, I would just do this:
- Find a large surface of uniform neutral colour: a white or a grey wall, that is, in shade (you must be in the shade as well). ("neutral" colour because CdS metres respond differently to different colours. SBC light meters should not have this problem, or have it very much reduced. Neutral is neutral anyway).
- Measure reflected light from a distance of at least a metre, paying attention to the angle between lightmeter and wall. Keep it orthogonal to the wall.
- Repeat measure with other cameras, and lightmeters, you have, from the same distance, paying attention to the angle.
You need a uniform wall because this way, the larger or the smaller the angle of reading, you can compare readings;
You need to be a bit distant, let's say at least 1 metre away, so that small differences in the angle of incidence does not play a role, and so that small lack in uniformity in the wall do not cause two different readings with two different lightmetre having a different angle of reading.
You need a neutral wall so that you can use CdS metres for comparison.
You need a wall in the shade so that it is more uniform, and there is less parasite light entering camera viewfinders (obviously you must be in the shade yourself).
All that said, you should have the same reading from all lightmetres, and maybe from all cameras. You might see small differences between lightmetres, but no more than 1/3 EV. I would be more careful in comparing readings from cameras, as those can be influenced by lens vignetting or lens flare (you can have slightly different readings when you change your lens) and by parasite light entering the viewfinder if you have the sun at your back.
I would not use grey cards, as a reading from a grey card is influenced by the angle formed by sun - card - camera. Grey cards are tricky, basically not very reliable, objects, unless you mount them on a tripod, stick them to a wall etc. so that you make the readings with exactly the same angle of reading and the same angle of incidence of the light. Also be careful not to project a shadow on the grey card if you use this method. With grey cards, a small movements in the card, changing the angle of reflection of the light, changes the light that is measured and this is possibly why comparing lightmetres is unreliable with grey cards, you can easily get small random differences.
A wall in the shade is more reliable because it is in a fixed position, so you only have to check that the angle of the lightmeters-cameras that you compare is always the same.
That's what I would do.
Fabrizio