Can you negate a positive without implying the opposite?

I don't generally think of good and very good as being different enough that I would want describe something as being between them in quality. That being said, you could achieve this effect by first stating the baseline as a fact, and then positing that it approached the next echelon, e.g.

That was good; almost very good.

You should be careful, though, because being finicky with a description in this way might come off as sarcastic.

Is there an "efficient" way to convey that sentiment? Normally, I would advise, "just choose the proper word," but you've hit on an elusive area that's a bit hard to describe in English, especially in just one word.

I can think of lots of ways to describe something that is "good": acceptable, decent, fair, adequate, satisfactory. I can also think of several ways to describe something "very good": excellent, exceptional, superb, first-rate, fantastic. But you're asking for a word to describe something better than the first group of words, yet not quite as good as the second – something that seems to fall into a no-man's land of the thesaurus.

One could say:

I'd give it a B+.

(Even though your question asked for something other than "explicitly outlining the precise measurement," I'm offering that suggestion anyhow, because I've often heard those words used to express exactly what you're asking for: better than good, but not quite excellent.)

As an example, from a NY Times headline: Obama Gives Himself a 'Solid B-Plus'

The food was good

and

The food was not bad

mean slightly different things.

I would use the latter to mean that the food was acceptable and reserve the former for the case when I really enjoyed it.