Pearl Sutton wrote:
If you have a bunch of them, put them in a line going toward the tree and you can figure out exactly how far away they need to be. I'd expect it to vary based on water flow off the tree (water flow may move some of the jugulone,) and possibly sunlight under the tree (crepe myrtles that are not shaded may have stronger growth in general.)

The only other factor that I can think may exist is the placement of the crepe relative to the Walnuts' roots. In the planting line or distance theory one crepe may be closer or it's roots may grow out to be closer to the roots of the Walnut resulting in it being affected faster than one that is closer to the Walnut tree itself but is further from any of it's roots. Bit of an unknown unless you have x-ray vision. Never the less, I would have a test subject on a root or near as you can for a while to see if it will be affected and I would go ahead and plant wherever you would like them to be. The testing etc. could take a week a month a year or several.
Life is too short. Make it as you envision it and Mother Nature will decide, those that survive will be beautiful.