I have been searching for a rule of thumb as to the approach to RUF. I was intending to extend everything proportionally, maintaining front end geometry.
I've drawn many RUF setups, and have concluded that on paper JrZook's Lil-Foot is the best ive seen (ive looked at a few, but not all... the information for ruf is scattered high and low).
It keeps the front end geometry proportional. The eye to eye of the ruf is parallel to the eye to eye of the original.
Because of the flipped rear hanger, he followed the profile of the chassis, which meant when the chassis was extended the correct amount the profile of the chassis remained unchanged. This maintained the geometry.
By not changing where the spring perch sits on the spring (i,e. didnt drill the perch), he didnt adjust the castor because moving the diff on the spring forwards or back affects castor because of the camber of the spring.
I didnt see any tech info on this, so I'm assuming he fluked it.

Effectively with people doing it other ways is getting the geometry all messed up.
If you just extend the chassis and add longer shackles, you reduce castor.
The loss in castor is the same loss as using your new extended shackle on stock springs.
The whole reason I was searching for more info (i didnt start the thread) was to find whats right, and as I suspected I feel the majority are wrong. Its the profile of the chassis which is neglected which affects the whole front end geometry. Currently theres seems to be no proper explanation as to why one is correct.
I was a bit sceptical of everyone saying its great, when I suspect there's clearly a loss of handling.
If someone has more theory again, please provide it. If you wish to provide a rebuttal please provide theory, or atleast something to justify you're argument.
Hopefully we can get to a point where it could become a proper how to ruf, as theres so much stuff on RUF, but nothing concise.