Guide Number vs Light Meter

[chassis]

To the OP, it looks like you have consistent results. You have characterized your equipment. Is it possible to use this information and continue shooting? If further accuracy is needed, testing with your chosen film and lighting setup would be one method. If the photo project is not ultra critical, I would suggest you are good to go.

 

[AgX]

But thus without walls and a ceiling or with them but with direct metering without dome (but calculating for the lacking opacity of it) one would likely get underexposure. (Reflections are part of the lighting that makes up the GN.)

I have to correct myself: Guide Numbers are established with non-reflective walls.

 

[Chan Tran]

The GN should be tested in incident mode but with a flat receptor and not the dome. I don't know if the OP meter has the flat receptor but if it does, please retest and you will find a little higher reading but I don't think it's as high as the GN would suggest. Flash manufacturers tend to inflate their GN.
Just checked the 308S manual. The OP should use the Lumidisc and not the Lumisphere. Using the lumidisc the OP should get about 4/10 of a stop higher reading.