I'm old school, old fart. Electronics are great, but the old mechanical ways can eliminate sources of error. If not done yet, verify TDC with a piston stop. To read advance beyond the markings on your timing tab, I would reccommend adding extra marks on your damper. If you turn the engine backwards until the timing mark on the damper indicates 10* advanced and use a center punch to make a mark at 0* (TDC), then move this mark to 10* advanced and put two punch marks at 0* and then repeat again adding three punch marks, you will be able to read to 40* advance with a simple timing light. Add a little white paint in the marks for better visibility.
Might eliminate a possible source of error.
Merry Christmas!!
Lindsay