'68 Charger with '70 440 Engine - Timing cover scale

[68XS29L]

Hey all:

I'm in the process of tuning my 68 RT that has a 70 440 engine in it.   Have a few issues, one is that pretty sure I have a vacuum leak that I need to track down.  The second is that I don't recognize the timing scale on the timing cover.   It's a little hard to read from the picture but the top is 0, then 10 - 0 - 30 at the bottom of the scale.   Shouldn't the scale have at least a 0 with 10 above and below for advance or retard of the timing from TDC?

Not sure how to set timing on this scale.  Any suggestions?

Thanks

https://drive.google.com/file/d/0B7eZWA1gov1oTHRfc29rTUQyWkE/view?usp=sharing

[heyoldguy]

It is actually zero on the top and then 10-20-30.