I would like to find out the angle between the Mars WS (winter solstice) location shown and the vernal point (0 deg) which is also shown between Pisces and Aquarius
in the diagram.
does anyone know how to find that angle?
It seems to me that the discussion in your previous post
http://www.orbiter-forum.com/showthread.php?t=37135 and the various papers there - particularly your link to the Mars24 calculations - give you enough information to be able to calculate this.
But before diving into the details, it may be worthwhile thinking about the definitions of the vernal equinox and winter solstice. To do that, we need to have in mind the concept of Mars-centric planes. The first of these is Mars' orbital plane around the Sun. This is Mars' version of the ecliptic. From a Mars-centric point of view, the Sun is always constrained to lie on its orbital plane - just as the Sun always lies on the ecliptic plane when seen from Earth. The second relevant Mars-centric plane is Mars' equatorial plane - a plane which is defined by Mars' rotation axis which is always perpendicular to the equatorial plane in exactly the same way as Earth's rotation axis is perpendicular to its equatorial plane.
These two Mars-centric planes - the orbital plane and the equatorial plane intersect along a line (the line of nodes). During the course of the Martian year, as the Sun moves along a path on the orbital plane, it will periodically pass through the line of nodes and cross Mars' equatorial plane. This will happen twice a year - once when the Sun crosses from below the equatorial plane to above it (the Vernal (Spring) Equinox); and once when the Sun crosses from above the equatorial plane to below it (the Autumnal/Fall Equinox). Between the Vernal Equinox and the Autumnal Equinox (northern hemisphere summer), the Sun is above the equatorial plane and, at some point in time (the Summer Solstice) its altitude above the equatorial plane is at a maximum. Equally, between the Autumnal Equinox and the Vernal Equinox, the Sun is below the equatorial plane and, again, at some point in time (the Winter solstice) its altitude below the equatorial plane is at a maximum.
Now, as with Earth, it is conventional to describe the position of a celestial body in the equatorial reference frame using right ascension(RA), [MATH]\alpha[/MATH], and declination, [MATH]\delta[/MATH]. These are essentially altitude-azimuth coordinates of the body in a planet-centric non-rotating coordinate system. Because the rotation axis is more or less fixed, and because most stars are very distant from Earth, the right ascension/declination coordinates of a star are essentially fixed. The position of the Sun can also be described in terms of right ascension/declination coordinates - but in this case, the coordinates change with time.
RA and declination are useful in the problem because (by definition), the Vernal Equinox occurs when the right-ascension of the Sun, [MATH]\alpha_S[/MATH], is zero (and the declination of the Sun, [MATH]\delta_S[/MATH], is also zero). And the Winter Solstice occurs when the Sun's delineation is at a minimum with [MATH]\delta_S<0[/MATH]. So, if you have a way of calculating the Mars-centric right ascension and declination of the Sun at any time, you can use numerical procedures to find the dates of the Vernal Equinox and the Winter Equinox. Mars24 and ancillary papers do just that.
An if you know the dates of the Vernal Equinox and the Winter Solstice, you can calculate the solar longitude [MATH]L_S[/MATH] on those dates. The difference in solar longitude between those two dates should, I think, give you your answer.
