You can get a fairly close value of the CG height by weighing your car at each wheel to get its distribution, then by raising the rear about 250mm the re weighing the front.
There are a few calculators set up online already, all you need to do is plug in the numbers.
CG Height Calculator
RC can be obtained by measuring
height of your LCA @ the chassis,
height of LCA @ the hub,
the distance between
the approximate angle of elevation,
the height of where the shock mounts to the chassis,
the approximate angle of the shock relative to vertical
If you feel you want to adjust your roll centres, your obviously in it for performance so you might as well have a go and calculating it and seeing where you stand, if theres little or lots of gain to be had. Just my 2c.
then with a set of drawing instruments and paper, (or a CAD program to make it easy)
you can draw a scale model of your suspension and work out the roll centre as such.