Hi Bob, I have used 18" on all my long bows. Exception is a couple where I did a bad job tapering for the fades and had to go back ending up with a 17"
These bows have varied from 66 ntn to 68 1/2 ntn. None of them stacked, even at 32" of draw.
I'm not sure what you mean when you say it is not "smooth" on the draw? Are you saying that less is working simply because the limb beyond the riser is a bit shorter?
If you truly are having some sort of issue with the draw, then maybe you are getting some stack? If you are getting stack on a 68" bow with an 18 inch riser, then I'm going to suggest that the problem lay in the profile of the limbs or in the taper of the lams.
More description of the problem would help.