Due to a post here about problems running 40 mph in 4th gear, today I tried a couple things. I slowed to 35 mph in 4th gear (flat road) it run perfectly good at that low a speed. Then w/ no downshift, I accelerated out of that w/ full throttle, which I don't recommend....because it did hammer perdy hard till I got to 45 or so. Did it again, at 35....accelerated easily and smoothly right on out of it in 4th gear.
These big singles still amaze me,.....I could loose 2nd and 4 gear, and still be perfectly ridable !

On to idle speed (rpm's) and a way to check it w/o a tach.
Mine is set to the hi side tollerance, IMO.
I rode along at a dead idle, flat road, and my speedo read 9 mph in 1st gear. If all speedos read near the same, I believe the speedo is a good way to check idle R's when the question is asked.
Maybe others will check the same, see what they come up w/, but I would say 8 mph would be to slow for these engines, and 10 mph would maybe too fast. But as always, your miliage may differ.

That method might depend on rider weight...

I'd just recommend that you set it to sound smooth, and steady.. much like a car idle,.. and avoid trying to be "Harley" cool, with a slow loping idle.  That slow idle will not get your oil pressure up good enough...

Try this... ... say out loud,
"put, put, put, put, put, put, put, ", as very fast as you can...
That's about the right speed... ...
... I think it really is!... ...