RE: How low should I get my idle speed?
Carl,
Listening to John Buckner is a good idea.
With that engine, I mostly use an 11 X 4 APC prop. That is true if the plane is one of the so-called "Trainers" or the World Models Sky Raider, which we use for RCPRO Club 40 Sport Racing.
If I want to do better aerobatics with the Sky Raider I use an 11.5 X 4 APC prop.
These props give lots of thrust for getting off the ground or out of trouble, but also help slow the plane for landing.
But, back to your original question. The idle should be low enough so the plane will descend and land. Try the 11 X 4 APC.
Ken Erickson