thanks HJ, I now see that the 1/8" slope is only for larger pipe.....
Section 708.0: Horizontal drainage piping shall be run in practical alignment and a uniform slope of not less than one-fourth inch per foot or two percent toward the point of disposal provided that where its impractical due to the depth of the street sewer or to the structural features or to the arrangement of any building or structure to obtain a slope of one-fourth of an inch per foot or two percent, any such pipe or piping three inches or larger in diameter may have a slope of not less than one-eighth of an inch per foot or one percent, when first approved by the Authority Having Jurisdiction.
So you don't get any advantage in terms of the branch arm length you can have by adding a dedicated vent stack to a fixture? My book makes it sound like "stack venting" is allowed, but since the pipe is doing double duty, you have to limit branch arm length. "..but such a stack-vented branch arm is limited. most codes allow 8' for 2" line, and 10' for 3" line."