No text, but a visual code

[HermesRevived]

I think your method is correct Antonio, and puts us in a position to work out ("solve") the text. We can proceed on the (well-founded) assumption that the Voynich 'language' is a development from the systems of astrological notation in use prior to the arrival of the standard zodiac symbols and the now familiar system of notation (which, as you say, prevailed after the printing press.) We can study examples of that old system of notation - where zodiacal divisions are marked by day-length - and extrapolate to Voynichese. The numerical glyphs in the Voynich glyph-set (like EVA [d]) are the place to start. 

The gallows glyphs are an innovation, though. In the Ashmole example you posted, the solstice is marked by the (+) cross. I propose that in the Voynich system, the author has developed a set of four glyphs to stand in these places. And the four benched gallows mark the half-quarter (or kerubic) days. It is a straightforward development from earlier notation, but an innovation. I suspect the Voynich system is built around that innovation. The chain of causation then is something like: 

*The author begins with existing systems of astrological notation.
*In those systems the solstices and quarters are marked (usually, and crudely) by crosses (or similar)
*The author starts by refining this and devises a system of consistent symbols to mark the four quarters (and half-quarters) of the year.