This will sound stupid but how are you accurate to 20 initial timing? I assume your using a different timing light than mine as the marks on the block only go +10 to -10
You can use that timing scale on the block (timing cover actually) as a scale to make more marks on the crankshaft than the one that is there.
1. Set the original crank timing mark to 0 on the timing cover.
2. Make a new mark on the crank lined up with the 10 ATDC mark on the timing cover. (10 degrees CW from the original crank timing mark.) This new mark will be your new 10 degree BTDC mark when it lines up with the 0 mark on the timing cover.
3. Move the original crank timing mark to 10 BTDC on the timing cover (move it CCW).
4. Make a 2nd new mark on the crank lined at 10 ATDC on the timing cover. (20 degrees CW from the original crank timing mark.) This will be your new 20 degree BTDC mark when it lines up with the 0 mark on the timing cover.
5. You can keep moving the crank another 10 degree to the left (CCW) and making new marks on the crank at 10 degreee intervals for 30, 40 and 50.
Or, just buy a stick-on timing tape for the crank damper!