What does the precessional parameter measure?
What is usually shown in cyclic plots showing precession is the precession index ($e \cdot \sin\varpi$ where $e$ is the eccentricity and $\varpi$ is the moving longitude of the perihelion). It is dimensionless ($e$ and $\sin \varpi$ being ratios).

Here is the precession index for the last 2 Myrs that I plotted using data from Laskar et al. (2004) (in the data file, first column is the age, second the eccentricty, third the obliquity and fourth the longitude of perihelion from moving equinox):


As for the range of values, by definition a $\sin$ can only take values on the range [-1,1], while the eccentricity (at least during the last 100Myrs) has values ranging from 0 to 0.067; hence a resulting precession index ranging in theory from -0.067 to 0.067.



References

Laskar, J., Robutel, P., Joutel, F., Gastineau, M., Correia, A. C. M., &amp; Levrard, B. (2004). A long-term numerical solution for the insolation quantities of the Earth. Astronomy &amp; Astrophysics, 428(1), 261–285.