1970 c-10 timing mark confusion

[retro dreamer]

Hey Guys, and gals

I know this should be straight forward, but i'm a rookie so please stick with me. I'm trying to do the timing on 1970 350 engine and all the research i've done tells me to set it between 8 and 12 btdc. I try setting it to the 8 the 10 and the 12 on the left side (passenger side) of the timing scale and its really alggy until i turn it past the scale to the left. What's the deal? p.s. i dont have a veci, or at least i cant find one..

Thanks a million,
Perry