In my opinion, nickle brass is a mistake, for this application. I own a 25'06 Ackley, and I use 270 Win PMC brass. Turning necks is not possible, even chamfering the necks results in chipping, if not now, later. I wouldn't ever use nickle, unless I lived in coastal Alaska, and had no intention of reusing the cases. However, nickle is great for handguns.
The method I used to fireform was with corn meal. I don't remember what powder, or how much, but the main thing is to decide what you use to plug the neck. I have tried bar soap, and didn't like it. I wound up using little 1¼" squares of newsprint, rolled into a ball and carefully stuffed into the neck.
Jerry is right about checking the action every so often for clutter. You need to blow it out with compressed air, or if you don't have a compressor, I think a can of air, like we use for computer parts would be okay?
I close the garage doors and shoot into a baffle box, but that corn meal eventually blew the back out, which was made of half inch particle board. I think it is best to shoot upward, and be careful when extracting the blown out case because some media may still be inside, and get thrown around inside the action if you do things horizontal.
If at all possible, the best way I have found, to fireform in one simple step, is to use a loaner gun with a junk barrel, the chamber of which is cut using the same reamer as is your barrel. Using this method, at the range, with surplus bullets and a standard, but relatively mild load, I was able to completely fireform 200 cases in little more than an hour.
By far the easiest fireforming, if you have the means, and a cooperative local gunsmith.
Good hunting. LB
edit: PS I have never had to use a case trimmer after fireforming to an Ackley. I check, routinely, but the necks never need trimming.