There's no way I know of to calculate the required castor angle - basically, if you make it adjustable (by engineering some way to move the upper a-arm fore-and-aft), and you leave yourself adjustment range from, say, 5 to 10 degrees, somewhere in there you'll find the right amount empirically (whatever is the right amount for comfortable self-centering of the wheel).
As far as bump steer is concerned (a change in toe-in during suspension travel changes), my understanding is that if you draw a straight line between the upper inboard a-arm pivot and the lower inboard a-arm pivot, the knuckles (ball-and-socket joints inside the rack boots) in the steering rack (BTW, the length of the outer arms is NOT a factor in bump steer, but IS in toe-in!) should center on this line. Other things are a factor too, of course, (for example, whether your rack is higher or lower than the steering arms on the spindles), but this one is the easiest to accomplish, and the most effective in reducing bump steer. I had to narrow my rack 2" for it to line up on this line, but moving the suspension fully through its travel results in no change of toe at all, ergo - no bump steer!