I used .409 because it is middle of the road. Sierra uses three BC's but I did not know that JBM would use three but it kinda looks that way. JBM also lists Litz's BC for that bullet and I have found that to be very accurate but I don't know what it is, I'm going to get his book on order ASAP.
If I used .409 in Loadbase and the library file in JBM they were within a 1/10th at 600yrds. With Loadbase I can lay out multiple tracks next to each other so I used .409 for one and another at .474 and they where .7 apart in the same manner, it sounds like you had.
I started this thread, feel free to take a run at it with JBM it is good practice and for fun.
Warm up your Ballistics Calculator!