All of what you say is OK. From what I remember on my 240 GLT, which came with the gauges, all the light grounds were tied together, and certainly you can tie all the light plus 12 together.
The only "problem" may be inaccuracies in the voltmeter reading. Taking the hot from the fusebox will give you whatever volt reading is at the fusebox, minus whatever volt drop you get in the grounds. If you run the meter return through the light grounds, then you MAY get an error.
I'd go with what you have described and see if the voltmeter reads reasonably. You could check it against the battery voltage with a VOM if you like.
If it isn't accurate enough for you, then you'll want to move the hot and ground leads of the voltmeter closer to the battery source.
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