Dont have ton of experience with 748r, only done one and it was a real challenge getting the offset map correct, it turned out real nice in the end though. Since you are doing it yourself time is no biggie and you can try swapping the injectors and see if it changes anything, just to exclude variances in the injectors, or have them bench tested.
Did a 996 that had 431 and G cams that I sent back since it was too far off to be healthy, base offset was over 30% more than horisontal, both at part load and full load, just to make it run properly, not ok.
You should not find a constant need for offset in my opinion, part of the range will but not the whole range.
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If you like to find out I can make an eprom without offset so youll get a clear picture of the difference?