Ask 10 people the same question and you may get 10 answers. 1 have a parellel 4 bar and I set mine at 2 degrees negitive realitive to the transmission angle. But it is adjustable and I can change it if I have problems when I finaly get the car on the road. You may not be able to change yours once welded. It is also important to know how much your rear end will turn as power is applied. That is why people use slapper bars on leaf springs set ups to minimize rear end wrap that would effect the performance of the driveshaft.
I think you may find additional info on Currie's web page and I think I have even seen an explanation on Steet and Performance webpage. Good luck.