I've never touched the low speed needle on any new engine prior to break-in. I set the high speed to whatever the recommended starting point is and go from there. I guess I've been lucky because all of my engines have started and run well enough to be tuned.

The OS-F and 13x6 is what I've used on my 70s. After more run time and fine tweaking you'll probably get around 2000 idle speed with no problems.