Finally had a chance to try it today. Not that hard to figure out. I just subtract any boost or turbo timing from the endbell.
So, I had the endbell set to about 25deg from my dyno results (just before current rises sharply).
I added 5 boost and 5 turbo degrees. When they both kick in that is 10 degrees, so I subtract that from the 25 and set the endbell at 15.