I usually try to keep my riser to about 1/3 the total length of the bow. I haven't built any real long one piece recurves but, use 20.5" in my 60" . It seems to have a good balance of speed and smoothness. I would like to see a picture of your adjustable for length press.
The very first bow I ever built used a press to build bows from 60" to 66". The form was half the length of the bow so, you glued one limb and half the riser at a time. One benefit of that method is the limbs were mirror images of each other.