I found this online:

The extent of the damage depends on how resistant a particular species is to juglone, and the amount of physical contact the plant has with the black walnut roots. Root contact, or close physical proximity to the roots (within ü"-ý"), seems to be necessary before the actual harm is done.

How big is the drip line of your walnut? If it were me, I would think 40 feet is plenty of distance.

According to Ohio State University Cooperative Extension, although there are many plants that are tolerant of juglone, there are also many that will not grow within 50 feet of the drip line of a black walnut.

http://ohioline.osu.edu/hyg-fact/1000/1148.html

Although fringe tree is not on most lists of juglone tolerant plants, Morton Arboretum does list it as such.

http://www.mortonarb.org/tree-plant-advice/article/887/plants-tolerant-of-black-walnut-toxicity.html