What length stems are you running on your normal (non 456) inbreds. I used a 65mm on my old 456 and a 100mm on the BMC i've just sold. Somewhere in the middle?
the normal inbred has the same effective geometry as the 456, just adjusted for a shorter fork, so I'd say run the same stem as your 456, unless other things have changed too, like the front tyre width or the way you want to ride it e.g. more XC and less hooning.
I've got a 70mm on my 16" frame and I think it is a little short. I'm persevering for a while but reckon i'll swap it for a longer one.
I've got an 18" with an 80mm, 6 degree stem. Works well for me despite coming from a crazy-long old XC bike (120mm stem!). It's a nice balance between stretched and fun.
20" 456 here with a 80mm stem.
18" Inbred with a 110mm stem, nice and stretched out!
20" with 110 which was the same on both
my inbred and 456
Use to run 70mm on my 16" and use to find it spot on.
70mm on my 18" rides well.
100mm 10 degree, upside down on my 20"
I've mostly used 90mm or 100mm stems on my 18" Inbreds.