I'm not sure about the 6.4 but I read on efi live forums that the goal is 40-50 percent of the duration of fuel btdc. I've done some calculations and I found 1 ms duration translates to 6 degrees at 1000 rpms, 12 at 2000 and so on. So on a 2.0 ms tune at 3000 rpms, the fuel would be injected for 36 degrees. I bet you could make a table in excel to calculate it for all rpms and pulswidths. Although this doesn't Take into account the piston bowl design or injector angle