we run a couple of mk2's predominantly in dirt events. Both have signs of excessive sideways diff deflection with rings scraped on the tailshafts from picking up the handbrake bracket & the seatbelt bolts.
What are your thoughts on a watts link v's a panhard?

If the watts pivot is attached to the chassis, how low does the pivot centre need to be to maintain or improve/lower the RC. My concern is too low and i'll destroy it on the first rock or rutted track.

Both cars run 2 leaf rears + the smallest leaf, konis & starion diff, billies & Hilux. Tyres (185/13 dunlop) which sit just above the guard lip

Watts... Boys running the panard usually rip the mounts out...

No comparison, watts every day of the week

would you attach the pivot bearing to the diff or the chassis and at what hieght?

depends on your suspenion height. and also where you want your roll center.

suspension hieght has the top of the taller 185/13 dunlop rallies just above the guard lip. RC I have no idea. My priorities are diff location & not having a negative impact the handling. Any gains are a bonus.
Is their a formula/method/rule of thumb/link which can help determine RC

Accurate suspension in Brisbane is the place to ask.
Have the bars running parallel to the ground, when the is leaf springs are under load.

I have been runnig a home made Panhard in the Autocross car for years, and combined with the homemade radius rods, it works quite well and I have never ripped it apart... PS - if you rip a Panhard mount off, you would alos likely damage or rip off the Watts mounts too....

The Watts is a better design with less compromise though, but more fabrication..

I've been googling and below sums up what most links infer.

http://www.billzilla.org/suspensn.htm
If it has a live axle, then the roll centre is often right in the middle of the diff housing but can be moved by means of a Watts link, Wobb link, Mumford link, Panhard rod, Jacob's Ladder, etc.

http://www.minitruckinweb.com/tech/suspension/0606mt_rear_suspension_mini_trucks/pinion_angle.html#ixzz1PWyimeGg
On a straight-axle-type rear suspension the roll center is "set" by whatever is used to eliminate lateral movement (e.g., Panhard bar or even a triangulated-type link system).

Does anyone with an adjustable Watts find thay vary much from centre ?