I think the debate about old part of the plant vs green tip misses the point.  I've participated in that discussion elsewhere on the internet, and at the time I was in the "old part of the plant" camp.  But what I now see is that its not that important what part of the cactus is used.  If its a good sized, potent plant then both mid and tip cuts will be potent.  And some older sections of a cactus can have corking, which is not good for potency (since the green, outer flesh is no longer present).  It should be a green, healthy piece from anywhere on a big, mature plant.  I think thats what will give the best results. 

I wish you all the best with growing your cacti Pete.  Thanks for taking the time to share your results with us.