Are you sure about this. It seems suspicious. Advance/retard at revs is usually known, so you set the baseline at idle/cranking speed and factor in WOT advance/retard.
If it's a must, you read stories on S&F about engines running away, plus with no load it'll be pretty difficult to hold the revs accurately (bearing in mind your sphincter will be shaking too)
I've read on S&F about running motors no load and running away. Don't see how they'd keep running once the ignition was cut, but seem to remember it was John Marles or Jay Smith that said it. So I'd play cautious and do as gav says - stick a small shit prop on it to give it some load and guarantee that it then also has plenty of water.