Be aware that some dial-back timing lights will show a considerable difference from what the timing really is if there's an ignition amplifier box in use.

I have an MSD 6AL in my roadster along with a Mallory Uni-Lite.

The 462" engine was initially timed statically, then started and the timing shot with a Sears Penske non-dial-back timing light.
It was on the money so I did the 20 minute cam break-in from there.

Couple years down the line - and having re-adjusted the timing with the old timing light I bought a new Sears Penske dial-back timing light.
The timing showed 20 degrees off from what it really was.
(This was with the dial set at zero.)

I forget which side of zero on the timing marks the dial-back showed, but it didn't make any difference.
I set the timing with the old timing light and took the dial-back light back to Sears.