You don't have to have the engine idling exactly 650 - I doubt you could ever make it do that. I know that reading the handset on my Wolf 3D, the revs can vary by as much as 50 rpm aroung the idle point.
If the engine sounds like it's idling at somewhere near the right revs, then you can set the timing. As long as it's not racing or anything. Usually, changing the timing a bit can often change the idle revs anyway, so you'd end up chasing your tail - idling too fast, change timing, idling too slow, change timing...