you are going to want the expander bigger than .426 because 44 mag is a non taperd round. the 444 marlin is bigger at the base than 44 mag so you are going to whant a bigger expander. I think what you want to do is expand the entire cut 30-06 case straight (no taper) then run it back through the 444 marlin die to give it the taper. then run it through the 44 mag die to do the neck and seat the bullet. my prototype brass resulted in .456 at the neck and .465 at he base. I am thinking you would want a expander .009 larger than the bullet. my bullet measures .429 (on my cheap calipurs) so that should give a expander of .438. now I am assuming the brass is the same thickness all the way back to the base. it may not be and also all brass is not the same. millitary is thicker so if you are useing that then then it all goes out the window. I am using some remington for my inital set up. what I am invesiong in the finished brass is a nice straight taper from the base to the area were the bullet is seated aprox .450 back from the mouth.
The 7.62 has two removeable expanders so it shouldn't be to hard to turn another to .426 (size of the 44mag expander).