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Actually, it turns out my optimism about your Poussin solution was a little premature. It looked good on your web page but when I tried to reproduce it with my autocad program the problems revealed themselves.
Though you can make a 7 x 5 grid out of the image-bearing part of the canvas, at least close enough to be plausible, if you start an 18.4349488 degree downward line (which is what the true angle of your left line is)from the top of the first vertical grid line to the right of the left edge of the canvas it will be way to the left of the left staff. It's nowhere close to lining up with the staff. It does pass along the right side of the bottom end of the left staff, but since that is the only part of the staff that it matches up with it would be hard to make a case for it being of any major significance.
It's also not possible to start a line from the top of the 4th grid line and have it align with the right staff. I didn't notice it when I viewed your web page but the line you have for the right staff doesn't really align with it. It crosses over it, starting on the right side of the top end and ending up at the left side of the bottom end.
The angle produced between the two lines the way you did it is actually off from the true angle required to match up with both staves by a little over 2 degrees.
Surprisingly, it IS possible to produce a line very close to the required angle to match up with the right staff by having the line from the 4th grid line terminate 13 horizontal grid lines down the third vertical grid line rather than 9. The line is still a little to the right of the right edge of the staff, but at least it is at the same angle, or about 1/5th of a degree off.
In summary, although the 7 x 5 grid may have potential, making the lines to match the staves the way you did is just not feasible.
Sorry to be the bearer of bad news, but somebody else would have checked it at some time anyway.
Schellenberger and Andrews do a better line up.
The right hand shepherd (the red shepherd) forms a seven sided polygram inside the same circle.
The key is the central image.