Yes each mark is 2 degrees. But in your drawing, what is "Off"? Also, your mark at idle is drawn as WAY ATDC, retarded, and at rpm, even further. I think you drew it upside down. As you rev, the timing will advance, move further from zero, BTDC. You know to time it with the vacuum advance disconnected from the distributor and plugged? Set it at about 12 deg BTDC as a starting point, then adjust it from there when it's up and running. A modified SBC likes initial timing, some as much as 16 or 18 degrees., up to about 36 total, as long as it doesn't detonate or "ping". Watch this video and see how the timing moves.