Saito tells you to keep it under, I think, 4000 rpm for the first 10 minutes or so of running.
I guess I'd say, keep it running at as low an RPM as you can. That would be under 4,000.
I do have a tach that I bought a few years back. I haven't used it in a couple of years now. The only reason I ever DID use it was out of curiosity.

Dennis-